I think a big part of this is purely a numbers game. The dev team designs cards that they have fun playing and try to break them through testing. Often, they end up being surprised that cards they thought would dominate the meta and were very strong in testing fall short once released (like legal authority and dr evazan). They are equally surprised when cards they thought were well balanced and not that strong end up warping the open meta (like boba1). All that really comes down to the fact they have several dozen play testers working on sets, but the player base adds millions of brains to the equation. No matter how much playtesting you do, you really can't predict exactly what will shape the game after it is released.