High scores bouncing around for no or little apparent reason once a score goes past the mid-700s is a known issue.

The main reason for this is the FICO scoring algorithms are biased toward the mean. All other things being equal, Fair Isaac's computers would "like" everyone to have a FICO score between (roughly) 670 and 720.

This is why it's easy to get out of the 400s provided you can get a couple revolving charge accounts reporting in good standing with low util. You can rise from 450 to 525 in just a few months, and past 600 in a year to eighteen months. But as you get closer to that mean, progress slows.

The opposite principle works for those with unusually high scores, say in the low 800s. The system "wants" to pull you down to the mean, so little teeny things like a change in credit ratio or higher util can punish you far more than someone whose score is 710 or so.