I'm not sure if I have this right, but it has been my experience, so here goes:
-Write frequently, at least once a week. Use good keyword research, make it legible, make it unique, etc., etc. When I was writing several hubs a week I noticed my score continued to rise.
-When I go on a following 'spree' or answer/ask too many questions, my score seemed to drop.
-Also, when I hadn't had any activity on Hubpages for a while, my score would often rise. My guess is that the most significant factor is how well your hubs do. I haven't done the math, but it looks like an average of all hub scores.