A lot of community activity can make the score lose a few points, so I'd think if you were at one of the times when you score was in the low 90's and lost a few points for "extreme" activity, that could explain it.
I've noticed my own usually goes down a point or two after writing a Hub, and if I spend a lot of time, say, answering questions (my "thing" when I don't feel like writing a Hub) and maybe making a couple of forum comments, the score seems to drop a few points. (I've always kind of assumed it's the system's way of keeping one Hubber from seeming to sort of monopolize the site (even if it's really just a case of the Hubber have some time off and wanting to waste it (lol) ).
All that aside, there are also the people who follow all kinds of people and end up with a score of zero, but that kind of just also goes back to the "lots of unusual activity" for one user" thing.
With traffic apparently also playing some role in individual-Hub scores, I can see how decreased traffic (if someone has it) could bring down the overall Hubber score (maybe - just guessing, though). That could explain why someone once in the high 90's might now hover in the low 90's, and have less of that margin of error for losing a few points because of something like a lot of activity. (Again, just guessing. :) )