I'm pretty new here and still figuring things out, but my understanding is that they appear to be based on the quality of your Hubs and your engagement with the HP community through your own Hubs, comments, questions, forums etc. I think the content and quality of your Hubs and how engaging they are all help.
The impression I have got (and the approach I think I am going to take) is not to worry too much about them. Of course a high hub score is nice and makes you feel good but they will change over time sometimes going up, sometimes going down. I think focusing on writing good quality hubs that you are happy with and connecting and engaging with people about your own work, their work and issues that interest you should be the focus. Your hub score will probably improve the more you do this anyway!