this must be why: "Towards the end of the 2.0 patch cycle, classes began to exceed the baseline damage values that we intended. In order to get back to where we wanted the damage output to be, we are lowering the damage output across the board. Players at level 55 will find that their damage has dropped, while players at level 60 end up having about the same amount of damage output that they did pre-3.0 at 55."
so in effect you kill things slower so they can hit you more so if more then one mob is attacking you then it takes longer to kill each so they are all hitting you...