Only making the expected return per next-share-contributed be totally independent of the pool history/round-lengths can strategic entry/exit be made profitless. One way to do this is to pay a fixed amount per share no matter what the relation to successful blocks.. but that requires the pool to have a reserve and insure the long dry spells.
MaxPPS is like this, but makes the "reserve" per-user so they can't cheat the pool by withholding blocks.
So it shows as earnings, but they're on hold indefinitely until you work some more? An amount held hostage to ensure loyalty?