The score fluctuates based on many things which, mostly, we can only guess at. Hubpages doesn't release their algorithm since this would leave it wide open for spammers to change their hubscores at will.
Some things that (I believe) effect the score are: -Traffic -Backlinks -Quality -New Content -Non text content (media, photos, polls, quizes)
However hubscore has absolutely no effect on your hubs chances of ranking in google or the money you can make.
The only thing that I know they effect is: 1. Captcha code requirement before creating a hub. 2. Dofollow links. A hubscore lower than 40 will turn all links on that hub to nofollows for search engine crawlers. An authorscore of below 75 will do the same, but across all your hubs.
And yes it can fluctuate hourly. I mostly ignore it now.
Pretty much without exception my Hub score will drop two, three or more points within an hour or two of first posting a Hub. Then it gradually, eventually goes up again. For example, today, March 31, 2011, I posted a...