Probably because they played 100 times this season? Tho it doesnt matter much that H2H, DelPO victories worth much more then Olderer ones.
Yes, Djokovic actually had a better H2H against Del Potro than Federer did at the start of the year, but this year's results have changed that. Federer has improved his winning %, Djokovic has worsened his. Not that there is much difference. I just thought it really unjustified to claim Djokovic can own Del Potro while Federer cannot. Del Potro can beat both of them, but more often than not he'll lose.