Time is also important in addition to robustness.
I'm a 632 and have almost 3k games in the system, all within the last 3 years. Earlier this year, I had a three week span where I won 8-2 against a 640 (an 860 performance), 8-6 against a 680 (a 725 performance), and 7-3 against a 700 (an 820...