ok, it must be variance.
block 20642 and block 20656 round length was exactly the same ( 1hour 10min )
in the same round time I had 10.000 shares less in round 20656 and 20% less payout.
so I guess on top of the bad pool luck I now also have bad luck and a constant low variance

I wish Slush would have a user hash rate graph ( like WeMineLTC ) that would show the variance over time.