Sort:  

I actually think what is happening is that when you click to upvote, it just updates the current price. If the amount goes down but doesn't update, and then you vote, it is simply refreshing the post value and your steempower wasn't enough to make it go higher due to other factors that forced it lower (usually because more funds from the "payout pool" have been split between more people).

I agree with that. I believe that's whats happening as well.

I've seen it said that this is because of the curation reward being paid to others, but not sure if that's it.