J.S. Matthew's answer and link are excellent. I have paid careful attention to the ups and downs of the past few weeks and a few patterns have appeared to me for both HubScores and my HubberScore:
First, the popularity of a subject can change over time. My Christmas Hubs have sunk slowly over the past two weeks, even if they are receiving some traffic; but I can tell that they are not being accessed now by topical searches on HubPages, Google or Yahoo. Other Hubs have higher scores now than they did two weeks ago simply because more people are doing more searches on these topics.
Second, I have noticed that more relavant links included in your Hubs will give you a higher starting Hub score, as will keyword density that makes it easier for AdSense to attach relevant ads.
Of course these are only a few of the things that calculate into the equation. See J.S. Matthew's link for other answers. Remember also that your involvement on HubPages--reading, commenting, and so on--will effect your HubScores as well as your Hubber Score.