Okay, that is pretty interesting!
I can’t square in my brain how this is possible. The difference between 973 and 971 remains quite small though, although it’s weird that there even
is an inconsistency, if you can call it that.
If we could identify the records of the managers on the tails specifically and not universally, we would have some interesting data. In other words, if the left (negative) tail was filled with outperforming managers, and the right (positive) tail was filled with underperforming managers, then your conclusion that good managers will underperform expected wins would obviously hold true.
Having said that, the number of times good managers have negative run diffs and lesser managers have positive ones could make seeing the forest through the trees a little difficult.