When I was a kid, I was very confused by the names of Fleetwood Mac band members. It really threw me for a loop that they had a Lindsay who is a dude, and a Stevie who is a girl, both counter to my intuition about names.
Maybe, I guess. That would be one of those "abbreviations" like "Peggy" ("abbreviating" Margaret) that doesn't actually abbreviate the name, and you just have to know that someone out there thinks it's related.
"Stevie" for "Steven" follows the ordinary rules of English. For "Stephanie", you'd expect "Steffie".