I think it was the total number of votes in a period (never worked out what a period was though). So when people had seen the image they stopped voting for it and it slowly moved down the list. Of course there were people who tried to get friends to vote for their image to keep it at the top but IMO that is just sad and nothing for an artist to be proud of. 

The top 20  also gives you something to work for. If you have produced a particulary great image using a less used package there's a chance that it would get into the Top 20 because lots of people voted for it within that category. With the Art charts there's little hope since it spans all the packages and mediums. 

Keeping an eye on the Top 20 every few days ensured that you never missed any amazing images that were posted.