My payout was lower by 18% relatively to the proportional number of my shares.  A possible explanation to this is a sudden rise in the pool hash rate towards the end of the block, which decreases the payout of all the other miners (but at the same time make a significant contribution to solving that block faster).  Let's try to illustrate this with an example: 
The pool hash rate= 1800000 GH/s  Your hash rate=200 GH/s your expected reward=0.002777778 BTC
Now assume that Miner X joined the pool with 200000  GH/s a few minutes before the end of the block, due to Slush's 'anti-hopping' algorithm, Miner X's contribution receives almost full 'weight' despite occuring towards the end of the block, so the increased short-term hash rate at the end of the block is now 2000000  GH/s, your reward drops by 10% to 0.0025 BTC.  This is not due to any 'fault' at Slush.  Actually it might have been caused by a technical or other problem on one of the competing pools, which caused briefly a massive shift of miners to Slush as their 'backup pool'.
Just my 2 mbtc.
Enjoy the recent burst of solved blocks, which more than compensates for that 'disaster'...

Cheers