how come I just got a master ball when I first met professor juniper at mistralton city in white 2
This is the special master ball that you are guaranteed to get in the game
Because GameFreak wanted it like that. Not really much to say.
No one knows why. Save it for later, or use it off. You get one later anyway by defeating Corless post-game. Maybe for the newbies who find it difficult to catch a legendary.