The dungeon finder prefers the instances that are colored yellow for you in the specific instances list. So yes, around level 30 you will be running a lot of SM Graveyard. That said, I have used it from level 15 on and I have done Wailing Caverns five times, with zero Deadmines, although they are in the exact same level bracket. So there might be more going on.

One thing that could influence it is that if someone selects a certain instance from the specific instances, it grabs people from the random queue to fill out the group. So if half the players in the bracket want to run a specific instance, the odds of getting other instances with the random instance are small.