Name Question

Hi Everyone,

I'm wondering if anyone might now the answer to this question.  Here is the background.

I'm researching the surname Guenther.  I've discovered that my 4x great grandfather's name is Peter Guenther.  He married a woman by the name of M. Elis. Theisinger.  I've only found it abbreviated but I'm assuming it is Maria Elisabetha or something similar.

When I typed in the last name Theisinger with Peter Guenther in my familysearch.org search engine a marriage record came up in the town that I know my family have been a resident of for generations around the time that would make sense.  However, the man's name is Jean-Pierre Guenther.  I'm wondering how come Jean-Pierre was in Germany around the late 1700s/early 1800s.  Having a nickname of Peter would make sense which is what is listed on all of his children's baptismal records.

Has anyone come across a similar situation?  Would that make sense that I'm looking at my 4x great grandfather and he just went by Peter for everything else?

Any suggestions would be great.

--Michelle