If you go back and read the posts by dcsobral, he goes over some his logic. I am using the code he created to generate my list.
For Tiers, that is the total unique(I BELIEVE it's unique, it might be total) POI's which are eligible to be a quest poi of a specific tier, either tier 4 or tier 5. Why is this important? It may not be important for some people ALL, but for others they might not want to do the exact same tier 5 quest 50 times in a row and having more tier 5's helps with some of the boredom.
The top7/top15 are answered in this post: