Following is the answer given by hubpages team in faq section.
"We use this to determine the quality of individual Hubs, much like Google ranks popular pages. Many factors contribute to our rating scale of 1 to 100, with 100 being our top score."
"HubScore takes into account many factors and will change over time as data about its performance continues to stream in. It will also fluctuate quite a bit, seemingly at random, and that is nothing to worry about. The most important HubScore components that you should be concerned with include:
*Amount of traffic - including percentage from reputable sources other than HubPages
*The reputation of the Hubber - your Hubber Score and contribution to the community
*The response of readers to your hub - including comments, thumbs up, etc.
*The uniqueness of content - copying content already available on the web will be penalized
We reserve the right to change the factors used to assess a Hub’s HubScore as we continue to determine those that measure a great Hub. The bottom line is if you write original, useful content, your HubScores will reflect the quality and work you've put into them."