Hacker News Reader

Heat/trending of some sorts?

I think it is likely something along these lines. I up-voted this question and the percentage rose.

But I upvoted another submission and the percentage fell!

Did you have Javascript disabled? Upvoting with JS enabled generates an additional GET request for an image with the same href as the vote button.

Perhaps part of their security minded hacker detection system. ;)

If @0942v8653 is correct, stories can't be downvoted, then this sounds like the best guess to me. :)