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.
Interesting, so shares actually drop to 0 value if the round goes long enough? I realized they were weighted toward later submissions but not to the point at which early shares would be worth 0. I was expecting a logarithmic curve.
Between this and what the other poster suggested (that my difficulty is set too high) I think I may have pieced together the cause (yours) and the fix (his). Testing now.