If Stephen is supposed to be pronounced "Steven", why not just spell it, Steven? And, why isn't Steve spelled, Stephe?

Why isn't Stephanie pronounced, Stevenie? Why does it sound like Stefanie? Should Stevie Nicks's name be spelled, Stephie? Why are there boys named, Steve and why would a boy named Stephan not be called Steven?

If Stephen is supposed to be pronounced "Steven", why not just spell it, Steven? And, why isn't Steve spelled, Stephe?
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