The RAC formula is a bit dodgy, I never understood it myself. I think it's a kind of average of the work done in the recent past, but with no upper bounds, it keeps on increasing as long as you keep on returning workunits.
It might have to do with the time the machine takes to report the workunits, or the time they take to get validated... I'm just guessing. Assuming it's a kind of average then if the workunits take more time to get reported/validated your average doesn't go up as fast.

But don't worry about that too much. I never understood why they came up with the RAC thing when they could have done a simple plain weekly average or something like that...
What it really counts for us to brag against each others is the total credit. :P