Here's my understanding (and I'm sure its not exactly right, though its close enough for us to grasp the concept):

All post and comment values are ESTIMATES based upon the seven day average price of Steem and/or SBD. Consequently payout amounts fluctuate with fluctuations in the 7 day average price. Additionally, the estimate is based upon your estimated share of a pool of newly minted steem that will be distributed at your payout time. The number of minted steem is fixed, but how that fixed number gets divided up among posters, curators, etc. varies. For instance, suppose there was only one post and one vote for that one post during the entire payout period. That one post would then get allocated ALL of the newly minted steem, making it hugely valuable. Because the algorithm that estimates your payout doesn't know exactly how active the community will be in posting/curating over a given period of time, it can't make exact predictions. If the community has been more active in posting and curating than the algorithm predicted, then the fixed number of Steem gets divided among more posters and curators than originally estimated. The algorithm is constantly updating the estimation, so when the community is very active this can cause reward values to be diluted slowly over time prior to payout. So...dilution is actually a good sign since it indicates robust posting and curation activity (which should ultimately lead to higher Steem prices).