If you are wondering why your post rewards drop before payout there are two main reasons!
The first and most obvious reason is that someone might have downvoted you, this can bring about a dramatic drop if that person holds a lot of vote power. Some of the Steemit whales downvote posts which they feel have collected too much rewards based on their perceived merit of the post. Some might argue that this is unfair however that is how the Steemit platform was designed.
The second not so obvious reason is vote dilution. Besides the voting power related to the Steem Power held an individuals voting power is diluted with the amount of voting they do in a 24hr period, this is reset every 24hrs. So a whale might not have voted for any posts, found yours and voted and all of a sudden you see an award of $100. Subsequent to that they went and voted on 50 other posts. With each of those subsequent votes (within the 24hr period) your reward diminishes.
I may be wrong, but I think the average over seven days of the value of steem may affect the dollar amount estimate that is shown on posts as well. But thanks for answering as it is a question many people are curious to understand.