I've noticed this too and was initially puzzled by it, but it makes sense after a bit of thought. Every day there is a fixed amount of Steem available to be distributed via votes. Keep in mind the total number of votes across the whole platform will vary day to day, and constantly be changing over time. If a vote decreases in value, I think it represents the fact that some other people have voted elsewhere, re-distributing some Steem away from your votes and towards others. It's a massive back-and-forth process until the payouts occur. So if a vote comes in early in the cycle, right after a payout, the pending payout will look large and gradually lessen over time as more voting is done on other posts. Of course there is also a maximum limit to how much of the pie a vote can contribute, depending on the voter's Steem Power, so that plays a role too.

I'm relatively new here and still learning about Steemit, so take the above with a grain of salt, but this is my working theory for now.