I'm pretty sure estimated rewards are based on rank and total prize pool.

Each rank has a certain share of the reward pool (I even seem to remember seeing the breakdown posted somewhere on SN).

So there are only two things that change your estimated reward: the prize pool getting bigger or your rank changing.

You may zap or post like a champ, but if someone else is zapping and posting to a more champion degree, you might see your estimated reward fall.

At least this is how I think it works in my fuzzy head.