From what I understand, standardized spelling was uncommon before the 19th or 20th Century, including surnames.
Whenever censuses, births, deaths, wills, or other documents were recorded, spelling was very often phonetic and at the mercy of the recorder, especially if those being recorded were illiterate and unable to convey the "correct" spelling of their family names. Add that to the variety of dialects/languages our ancestors brought with them - in my case, mainly Welsh, Scots-Irish, German - and you get interesting phonetically spelled versions of the same name, which is what you're dealing with.
In some dialects, for instance, names ending in the vowel -a were often pronounced as -er and that eventually got picked in the spelling. I live in the South and I still hear names like my mother's, Glenna, pronounced as if it had -er - Glenner (which always made me cringe). If you've ever watched Doc Martin on PBS (US), you'll hear it in the (Cornish?) dialect whenever someone says Louisa, which comes out Louiser.
My Andrews ancestors were also off-&-on recorded as Anders and there's even a branch of relatives in my area who go by that instead. My great-grandfather Robert Burns Andrews' marriage license to my g-grandmother had Anders though they went by Andrews. I'm sure it displeased his bride to see the misspelling on their license since she was a school teacher by profession! The license recorder also misspelled their little community name of Orlando as Olando, so he wasn't a gifted speller anyway.
There is a country road in a nearby county named after my German-derived Bostian ancestors who once lived there. Unfortunately, the locals always mispronounced it, and it's called/spelled Boston RD. So I understand how frustrating it can be.