Pepe LeFrits
Reading up it's based on averages and I think exclusively on PL transfers too so doesn't account for Madrid and their galacticos. So I assume the average fee between Shevchenko being sold and Rio being sold didn't change all that much.
It would explain why Chelsea (and United) dominate, because their fees were probably well over the average in that era, whereas by the time City came along the average fee went up considerably.