The use of consonants and vowels are very different between the French and German languages and our english language. I believe that when our forefathers landed in America and were required to provide their names any number of things may have occurred. They wrote their own name in their native language or their own second language. A clerk wrote their name for them, spelling it in their own phonetic interpretation of how it sounded. The ancestor wrote it with his own phonetic interpretation of the english language.
Most researchers can point to verified spelling changes within their own line.
This could lead one to conclude they became more familiar with english has time went on or that they opted to spell the name in a common way to that of their relatives living close to them. In the late 1600's the Catholic Church decreed that only given names from the New Testament could be used. The single fact has been the source of considerable consternation for genealogists attempting to identify their ancestors.