Everytime a song is requested it adds to it's running total.

The Top 40 is set to go back I believe 21 days, but I'm not sure what I have it set at. I change it every once in awhile to shake up the list.

If it IS actually set for 21 days, then a song that was requested 21 days and 1 minute ago would have just seen it's number drop by one. It's actually exact like that. So throughout the day the number may go up and down.