Not sure what exactly you are seeing but it might have to do with the max payout on these being capped at $25. That feature hasn't been used much before and bugs or glitches in the UI would not surprise me.
More importantly though, if you are voting something already at or above the cap, your vote is being wasted since the payout is only going to be $25 anyway. If you want to support the initiative I would suggest looking to the "additional vote" comments, which seem to get get fewer votes, or possibly another top level post that is below the cap.
What he means is, when voting on comment or this post (while under 25$ reward) sometimes it shows that the payout drops, in stead of increasing. I do see the same, but after refreshing the page the payout value seems to be correct again.