Dps sims don't take into account things like time on target, buffs/debuffs brought, they assume perfect rotation.
Dps meters don't take into account that some raids are missing some buffs and that marauders/sents always receive bloodthirst and may show higher than specs that don't receive it, gear differences and are typically a small sample size. They also don't show average dmg, but only highest dmg, some specs have random procs that can mean they are either high or low and rarely in between (ie fire mages in wow)