As my "block estimate" currently goes down, I assume the pool has used up all its savings. At the same time, "Maximum reward" goes up like I expect my reward to do in a pure PPS scenario.
So maybe "maximum reward" is the amount of BTC we will be awarded in the future, if the pool has a few lucky rounds?
That would make sense... so I guess "block estimate" is wavering up and down because my share of the pool is changing, which normally doesn't matter under PPS but does matter now that the total payout has exceeded 50 BTC?