Post payouts fluctuate up or down based on the price of Steem,
and possibly on how much voting your upvoters continue to do.
For instance if you are the first vote when their voting power is worth 100% it is worth more to you, but if they went on to vote quickly on a huge number of other posts, it will lower their voting power.
I try not to lower my voting power much below 90%. Which is basically 40 full votes per day. Or it could be 80 votes at 50% power, etc.
What can lower your pay out is a flag. Depending on the flagger's Steem Power, it could bring you all the way down to zero or lower your payment only a couple of cents.