Below is an explaination on how the TOP listings work (the last 45 days and the "of all time") and the Todays Top works in exactly the same way only its on the last 24 hours (not based on the day the image was uploaded but on ratings, views, comments within the last 24 hours so that list can change all day long)

Top 100 Most Viewed - RecentThis dataset is calculated based on which images were viewed the most in the last 45 days. For any two or more images with the same number of views, the ones with the most recent last view are before the ones with the least recent last view.
Top 100 Most Viewed - All TimeThis dataset is calculated based on which images were viewed the most ever. Ties are handled the same way as above.
Top 100 Most Commented - RecentThis dataset is calculated based on which images have received the most comments in the last 45 days. For any two or more images with the same number of comments, the ones with the most recent last comment are before the ones with the least recent last comment.
Top 100 Most Commented - All TimeIdentical to most commented recent, except without the 45-day restriction.
Top 100 Best Rated - All TimeThe most complicated of the three, this is sorted first by the ratings of images. The higher the rating (5-star) the higher they are on the list. For any two or more images that have the same rating (and 99% of all best-rated images all have 5-star ratings), the image with the most ratings comes first. So, in most cases, the Top 100 is actually sorted by number of ratings since all the images that end up there are 5-star images already. If any two or more images also have the same number of ratings, the ones with the most-recent last ratings will come before the ones with the least-recent last rating.
Top 100 Best Rated - RecentThis one is a little trickier than just limiting it to ratings placed in the last 45 days because average ratings are calculated using a standard deviation, etc. This can't be done on a database level and used for sorting, so what we have to do here is pick all images that have been rated at least once in the last 45 days and then sort by rating, rate count and last rate.

In addition to this logic, Top 100s within sections, genres, and section/genre combinations are also limited to those respective sections, genres, and section/genre combinations.

Hope this helps:)