My guess on the matter is that many people had weird spellings of Drashti's name, like Drishti, Drushti, Dhrashti, and so on.. so I'm guessing Mishti is technically a mix of Mayank and Drishti.
But I'm glad it's not MaShti actually cause MaSh is MA SA .. and MaShti just sounds like an extension of that -_-
also Mishti means sweet in bengali, so I guess no one ever wanted to change it and it stuck!
It annoys me to no end when Maan calls Geet Mishti grrrr and then gurti fans try to name gurti MeesShti :|