I have noticed the same thing. I cannot figure out the method they use to rank stuff - I have 2 almost identical hubs (not content) but structure, with original writing on each, each is about the same length, and they are even on a directly related topic to each other. One hub has a ranking of 66, the other has a 50. This is after about 2 months or more, so I know its stable. The layout is almost identical, including modules. To me this makes no sense, as they even both have about the same low level of traffic.
I would think that continuing to add hubs would eventually raise your score, but I am not sure they care as much about that as they do in your actual participation in the "community". Try searching for hubs you like and commenting on a bunch of them and see if your score improves. As with any venture, do not expect a few comments in a single day to do much, it takes time.