Needless to say, that website will never be accurate. I saw hacks that can increase the number of registered users and online members (and fake all statistics you can imagine). One can do that for any desired script, even phpBB.
And even if it's not the case, most of the data recovered was not updated anymore in the past years, so a board featuring 90K members today is listed as having 50K, and in 3rd place, while the 1st has 75K users. Same goes por post counters. And the funny thing is, the 75K board have the same number of users since that time. I wonder which one of them were using the hack.
On the other hand, I would like to see those boards listed as having a good number of unique visitors each month. I think that is good way to measure the success of those pages, rather than the user/post/online counters.