That's probably happening because the last share submitted was during the beginning of that round, and there weren't any submitted toward the end of the round.

For example.  If I am mining with a nice video card and cranking out 1 share per second for the first hour of a round, then the power goes out and the round lasts for 3 hours, my reward will drop to 0, or almost zero because the shares are weighted to discourage pool hopping.  The shares that come at the end of the round have more weight than the shares that come at the beginning of a round.