Smogon's tier system is used to rank Pokemon into several groups based on their perceived power and usage in competitive play.
That was the Gen 5. Smogon's tier system has always been somewhat based on usage it seems, and Gen 6 is no different. The thing was, they had heaps of time to refine the Gen 5 metagame as they saw fit, unlike the Gen 6 so far. We aren't that far into it yet.
If you look at the tier names;
Yeah I think that kind of gives it away that it was kinda always based on usage. Anyway, I would guess Smogon lacks enough information to sort it out, and is just going on usage for now, or they're just lazy or something :P. They still shift Pokemon around right now, with examples being Mega-Lucario and Genesect, based on suspect voting and how overpowered it seems the Pokemon is.