In the United States, the spelling of words and names didn't settle down until the late 1800's when literacy became widespread.  Nor was the precise spelling of words even considered important until the 20th century, as long as the meaning was conveyed.  For example, our word "ton," meaning 2000 lbs, could be spelled ton, tonn, or tonne.  All three spellings would have been considered entirely acceptable to someone in the 17th, 18th, or even 19th century because they all represented the same sound.  Their attitude would have been that the purpose of writing is to communicate, and if your writing communicates your meaning, that's all that's required.  Anyone who has transcribed more than a few historic documents can tell you it is not unusual to find someone (even an attorney or county clerk) spelling the same word or name in more than one way — in a single document!

Words were sounds thousands of years before they were written down.  Human language is speech.  Writing is a feeble attempt to represent speech visually.  So, do not get "hung up" on spelling, particularly on the spelling of names.

Most of our ancestors were illiterate subsistence farmers.  Their names may never have been written down in their lives, except possibly in the church register when they were christened and later married — and on a stone in the churchyard when they died.  In our society, with general literacy and vast, cradle-to-grave record keeping, it can be difficult to imagine what it was like to live in a society where few could read and few records were kept.

How many times in your life have you had to give your name and date of birth?  Can you imagine never having had to do so?  Can you imagine forgetting your birth date?  But people did forget, and they forgot because they went a lifetime without it mattering:  no forms to fill out, no classes to register for, no tests to pass, no jobs to apply for, no driver's license to get, no income taxes to file, no insurance claims — no credit to apply for.  What mattered was raising crops or making goods and maintaining a home and caring for ten or twelve children.

Even a man as intelligent, literate, and famous as Jefferson Davis, President of the Confederate States, lost track of his birth year.  Jefferson grew up believing he was born in 1808, then, years later when he was an adult, an uncle told him he was born in 1807, not 1808.  For the rest of his life he was uncertain, and so are we.

To an illiterate person, their name has no spelling, only a sound.  When they said their name to a county clerk or census taker, how it got written down depended entirely on the knowledge and diligence of someone who may have been barely literate themselves.  And if the clerk was English and the subject German (or some other non-English nationality), the name often got slaughtered.

I can think of cases where, in one census year, all the persons of a particular surname in a county had their name spelled one way, then in the next census, spelled another way, then in the next census, back the first way.  These people were not changing the spelling of their name; there was a different person enumerating the census each time.  The same thing can happen when moving from county to county.  In one county, the clerk spells their name one way, in the next county, a different clerk spells it another way.  Let me repeat for emphasis:  by and large, the way you see your ancestors' names spelled in the records is not how they spelled it (or would have spelled it had they known how to read and write), it's how the person who wrote it down arbitrarily decided to spell it.

These arbitrary spellings — which prevailed until literacy became widespread — are primarily what we would call "phonetic" misspellings.  Clarck for Clark, Canady for Kennedy, Garrod for Garrett, Robards for Roberts, Witmer for Whitmore, Thaxton for Thackstone, Dixon for Dickson, and so on.  The greatest misspellings were of non-English names.  Many of these names were converted by English-speaking clerks to equivalent, or at least similar-sounding, English names:  Braün became Brown; Müller became Miller; Ihle became Ely; Kopf became Cupp.  In the case of given names, English spellings were substituted for foreign ones:  Sanna and Susanna became Susannah; Catharina became Catharine or Catherine or Katherine or Kathryn; Maria and Marie became Mary; Andreas became Andrew; Wilhelm became William; etc.  Other names simply acquired an Anglicized spelling:  Dreischmeyer became Dreshmire; Dannemann became Denman; Schantzenbach became Johnsonbaugh; Kettenring became Cotton; Schaeffer became Shaffer, Shafer, and Shaver; Abendschon became Obenchain, Ovenshine, and Ovenshire; and so on.

With experience, you will learn to not attach so much importance to spelling and to "roll with it" when you find a novel spelling because the important thing is to recognize the name regardless of how it is spelled — and to always try at least the common variations when searching via computer.

• Prior to the 20th Century, the given names Edward, Edmund, and Edmond appear to be virtually interchangeable.  Likewise, Esther and Hester, Helen and Ellen. 
• "Ellen" can be a nickname of Eleanor, which can also be Ella Nora.
• The names Henry, Harry, and Harvey can easily be misread for one another — and don't forget that "Harry" is a nickname for Henry.
• The names David and Daniel can easily be misread one for the other.
• Sometimes it helps to say the name out loud to recognize a name that is grossly misspelled.  For example, say, "Edathan," out loud, and you may recognize that the name intended was Edith Ann.

One of the best examples I've found of how German surname Straub — my mother's maiden name — became converted to it's most common variant, Stroup, is in the marriage bond of Jacob Straub / Stroup & Betsy Dillinger.  Note how Jacob signed the bond in German script, as "Jacob Straub," but the clerk recorded it as Jacob Stroup! 

My favorite example of spelling variation is a distant Clark uncle, whose actual given name is uncertain because all of the following variations have appeared in the records:  Reyderus, Riderous, Rydarus, Ryderus, Rideras, Riderus, Rideruss, Reydias, Ridemus, Iderus, Idarus, and Adarus!  Usually, there is a clue to its origin (e.g., the many misspellings of Apollonia, the feminine form of Greek, Apollo), but I've no clue what the origin of Reyderus might have been (anyone know?).

Farrell Littleton had his given name spelled correctly in a letter written by his employer's educated brother, President George Washington, but slaughtered by nearly everyone else as Pherre, Pherril, Pheril, Pherel, Ferrel, etc.

My own surname, Matthiesen, acquired it's second "t" when my great-grandfather immigrated to California in 1886.  In Denmark, it was spelled with a single "t."  Double-t is the English way, as in Mattheson or Matthewson.  At least we managed to hang on to the Danish -sen ending, which is typically changed to English -son upon immigration to the U.S.