To explain the slush algo in short:
You earn score with the shares you submit, and the later in the round it is, the higher the score.
This is to prevent pool hopping, that is to encourage you to stay with the pool even if a round takes quite long.

This leads to a possible scenario, where you submitted many many shares, but still get low reward, cause you have not been there at the end of the round.
This should average out quite fast if you mine constantly.