I'm inclined to agree with the "lost connection" answer.  21728 (where you got the bum deal) shows a small drop, while the next round - 21729 - shows a larger one.  If the lost connection occurred towards the end of 21728 it could account for the low reward due to the rapidly decreasing value of the shares already submitted, while the next round was long enough for the reward to recover after the connection was re-established.

It looks to me as if you were the victim of circumstances, such as a temporary issue with your ISP, rather than any fault in the pool.  A half hour's downtime could easily go unnoticed unless you are actually using the computer when it happens.