Koehn and Kern from Mecklenburg

Hello:

My great-great grandmother's maiden name was Koehn (but appears as Kerhn in her marriage record). Her entire family (mother, father, and three brothers) went by Koehn in some records and Kern (or Kearn) in others. In the Civil War pension record for one of the brothers, both surnames appear - even on the same page! I know they look similar, but they are pronounced so differently (I think). I don't understand why there's this variance. They are distinct surnames according to the Germany surname book. They were from Mecklenburg-Schwerin (not sure where yet, possibly the Stavenhagen area) and immigrated to Outagamie Co., Wisconsin. There's another Koehn/Kern family that immigrated to Wisconsin that has this same issue as well (perhaps they are related, I don't know yet).

Does anyone have an explanation for why this might be? Is it perhaps some regional Germany dialect?

Thanks,
June