I have noticed same thing. Looks like pool uses PPLNS reward system with very small N. Older shares dropped.
At homepage announced:
Blocks are distributed proportionally among valid submitted shares.
It must be a PPS i think. Pinpin needed for explanation.
(awaiting mining until...)
Thank you guys, I also received another such report from other user today. I will be investigating this during weekend. Perhaps something to do with clean up or so. I also noticed it happened only to some coins, not all