I wish I remembered where I read this, but I recently was browsing the site trying to find that answer directly from hubpages rather than a person. If I knew where, I'd send you the link, but it said that it matters your quality of hubs, how much participation you have with other hubbers on your hubs, as well as, if you participate in the forums. People rating your hubs up and down will effect it as well.
Aside form those things, I'm not sure. My score suddenly dropped a lot, and I'm not sure why? So I wish I had a better answer for you. :)