Jen, Colin asked the same question in another thread. It's not an exact science, but just roughly depends on how many entries overall, and how many in a particular level made it into the top ratings overall.
In this case, 12 of the top entries overall were from Level 4. So although I would normally have only made it the top 10 for Level 4 because of the number of entries, as I did with Level 1, it seemed kind of pointless to do that with 12 of those entries being in the top 15 overall. So to be nice, I added the next three in the list for Level 4, which is why there are 15.
In Level 1, no entries made it into the top rankings overall. So we just had the top 5 for that level, as well as another 5.