When you first write a hub and publish it right away, the score starts out low. I'm not sure of the exact score anymore. At one time it was 45 then another it was in the thirties. But there is a general area between the two where new hubs will start out.
As hubs are left hub and age, they receive traffic, get new updates, get voted up and are given feedback. All these things contribute to the score, though the precise algorithm for determining such numbers is unknown to us mere hubbers ;)
Your newer hubs, likely, just haven't had the chance to mature yet, the scores typically go up as they age. Also, the advers can occur with getting voted down. It can cause your score to drop some, as well as losing out on traffic. As I said before, the exact formula is a big mystery but, other than tweeking and updating and generating traffic to your hubs, there isn't much you can do to affect the score. As long as you are writing quality hubs, they will continue to grow.