Even when considering all of the above, its possible for cgminer and the pool to not quite match. Long story short, don't go by the immediate averages, look at things a little more 'big picture' over the course of an hour or two.
GPUMAX can also show slower hash rates if the stats calculations are running behind.
I think Pirate is working on some things on the back side of the server. Because the stats on GPUMax are totally borked right now. I'm guessing it probably has to do with the load balancing.