Very good question. I guess the administrators are constantly testing and upgrading the system. Because Hubpages is a "Social Network" that means your score is dependent upon many factors. Part of the tabulation is how often you visit and participate.
That could mean writing hubs, commenting and rating other hubs, interacting on the new forums and answering questions. I'm wondering if it could also include linking to other people's hubs within your own?
Anyway, this has always been a a mystery to me, as well. I'm sure some top hubbers have written about it over the years.