Sophie is not a nn. It is the French form of
Sophia. Just like Marie/Maria, Natalie/Natalia, Julie/Julia. In medieval times, the "e" on the end of the word was more pronounced. The sound is the same as in the English word "could" or "should." So there wasn't that much difference between
Sophia and
Sophie. In modern French it is hardly uttered. (At least that's what my French professor M. Maubre taught me in college.)
That's not to say that
Sophie couldn't work as a nn for
Sophia. It just bugs me when people think of it "only" as a nn.