It's not just lower ranking reviewers or new reviewers. When I first hit 7, I was bouncing between 7 and 6.9 for weeks, until it stabilized when I wrote a few more reviews.
It DOES depend on everyone else on the site, like Sam said. So, if a lot of people are really active and you're having some down time you may drop a bit. Even the highest reviewers, in the 9+ strata have their ratings move around due to activity, review frequency, others' activities, votes and comments as well as other conditions.