Most of the ranking weirdness you see is caused by the way the current day is added to the scores. If it's early in the day, it's not very competitive so on a relative basis some people are high value. As more people start gaining value it "normalizes" everyone's score for the day.

It's intended to mean that for the given time period, that person added the most value, which we define as zapping well and posting/commenting stuff that people zap. Value is the word we chose for rewardability.