It's showing groups which have an abnormally high rate of users joining, based on that group's history. The technical math is "number of standard deviations above/below the mean"
And as it turns out, it's kind of crap. So yeah, lesson learned.
I'm sorry that you have to see things you don't like on your dashboard, but please put up with it for now. We're working on some major improvements in V3, so this will hopefully no longer be an issue.