As @ArsenLapin1 pointed out, the average winrate doesn’t show if there’s lots of both too low handicap and too big handicap, because they cancel out each other.
Maybe it would be interesting to see, how well the rating system would do if it was betting on itself. It does make finer distinctions within the range of one rank, so it could say “I expect black’s chance to win is 45 %” and if black wins it would get 0.45 points. If white wins it would get 0.55 points.