The reward you see is an estimate, because the final payout distribution is not known until the final moment.
There is a set reward pool to be distributed, and it is spread around based on upvotes and downvotes (weighted by SP). The more upvotes there are in a pay period, the less each one is worth.
For example, if there are only 10 upvotes this week, each upvote is worth 10% of the total pool.
If there are 100 votes, each one is worth 1% of the prize pool.
If there are 10,000 votes, each one is worth .01% of the prize pool.
So what you are seeing is that other posts are gaining upvotes at a rate faster than your post is getting upvotes, so your relative fraction of the total prize pool is diluted.
Jess. Oh ok now i comprehend ...thanks alot sir