Hubs with very good content will earn the 100 rating. Traffic, visitors' votes etc will push up the score, but it may not reach 100. And I think Hubpages staff (or automated program according to certain criterias) is pushing the good articles up to 100 scores, so as to attract more traffic to Hubpages.
I have a few hubs with exceptionally good traffic, but its scores lower than other hubs with much fewer traffic.
I believe Hubpages will help you to push up your score (to attract more traffic) if you write really good articles.
On the other hand, generally, upward move in score mean increase in traffic.
Just my 2 cents of opinion and observation.