Here's how it works
They have their own bot, kind of like Googlebot but just not so powerful.
The have a huge of index of forums right accross the range, from VB to PHPBB2/3 SMF even things like Fireboard are also listed I think.
Their bot then from what I can gather works down the list caching every forum for its stats.
This happens (from what I can gather also) about once an hour and updates the list.
Just happens that at the time I checked Peter what on top
So the stats should technically be called "Top authors since 12PM Sunday night - updated around an hour ago.."