Beside the gear another issue is that when yer casting yer lead starts casting before yeh other chars which guarantee that his spell pretty much always lands where's as any othe ones that lag behind,
when it comes to overkill situations leader spell hit kills the critter the other chars spells dont land which drops there dps.

This is assuming yeh have all the macros the same.

Hope that made sense hmm caffeine wearing off ... time for more cooffeee ....