The amount of reward you get per vote is based on the vesting Steem of the person voting, and the more they vote on different things within the same 24 hour period, the more the value of each vote gets diluted. So after voting on your post, if someone moves along and votes for a few other posts, the value of the vote they originally put on your post drops.
The specifics are here:
https://steem.io/getinvolved/posting-rewards/
Scroll down to "Limited Voting Rate".
RE: Why sometimes steem reward go down?