It's based on power .
I know this because most of my opponents are high 70s or 80s, me being a 85, but there is one and only one lv 64 that I encounter now and then, and she have MUCH higher than normal power.
This is, of course, talking about breakthrough and 10-12 node, as I don't pay attention to my sweeps. The lowest player other than 64 I get is about 75.