I'm no expert, but I think it's the extra credit stuff. The pool recently has very bad luck and does not have coins to pay everyone in full. It will give you some and remember the extra credit for your address. When the luck will get better, you'll get more coins per share.

On the graphs, you can see the blue line what has been paid a the pink area what should have been paid.

The explanation of the reward system is here: http://eligius.st/wiki/index.php/Shared_Maximum_PPS