That's because of the score based system they use at that pool. The newer a share is, the more it is worth. The older a share is, the less it is worth. On long rounds, old shares can become worth very little (or even worthless).
Yes, I am mining there for almost 10 days now. Total 2500 MHash/s power but the score system is kind of messy. When we start a new block, the score starts to raising over time and after like 30 minutes I reach several millions in score but a minute later it is back to 0. I am sure I have no disconnects. From what I understand from Slush's info, it the more time you mine (without disconnecting) the better score you get and I just cant get it why it gets to 0 over time. For longer blocks mining like 1-2 hours and more the score may reset more than 10 times. Maybe I am missing something but the way it looks to me, it is not good.
PS Yes, I understand the point of score system to fight cheaters but just think there is something wrong here.
PPS From the pool posts Slush seems like a great guy and can't say anything against him