This is CAPPED PPS. Essentially, each share you submit is an "IOU" of the straight PPS value. But the pool never guarantees it will have the funds to cover those IOUs. Eact time a block is found, it proceeds by paying the most recent first shares first. If the pool is unlucky, the block reward won't be able to cover all the shares that were submitted for the block, so some of your shares will be unpaid. If the pool is lucky, it will have money left after paying every share of the block, so it'll be able to pay some unpaid shares from a past block.
The "estimated change" is the value you would earn from a given block if it were found NOW. Of course, it is getting smaller over time, since, at the beginning of a block, if it was found now, you would be getting more that the straight value of the shares you submitted for the block. But it will converge after a while.
Since it is not PPS, you cannot expect a constant payout, even if your hashrate is constant; you need to check your payout averaged over at least a week to see if it makes sense.