Actually Filth is right more or less. Statistically it really matters, how big the number of players for each specc is. Those parses on WoL etc. are often very depending on procc luck, raid buffs, raid dps etc. RNG is very important. If you have 1000000 parses of affliction and only 1000 of destro, its very likely that you can find lots of very "lucky" affliction warlocks which will rank. As an example for this phenomenon you just have to look at arcane mages after they got buffed. According to raidbots they went from lower middle of the pack to #1 within two weeks. Sure, they got buffed but those buffs didn't double arcane dps. It's just a statistical phenomenon...