I haven't played WoW for quite some time. Although I have been playing other MMOs that require a lot of number crunching and maths. EVE, Anarchy, Starcraft.
This isn't going to be a long-winded thread involving maths however, just a simple observation.
Many classes have abilities that increase damage at certain health periods. (Above 80%, Below 20%, etc.)
The longer these periods are for certain classes, albeit modified by the boss' abilities or however, (reducing the damage it takes, healing itself, increasing the damage it takes) the more chance said classes have to do more DPS than the other classes.
I'm not sure which classes are top of the charts currently so any insight would help!