Because they were exceptions to the rule.Why was YPA not a predictor of how Tom Brady would be? Or how was Marinos second year YPA almost 2 yards above his lifetime average. Also of course Drew Brees second season YPA was a great predictor of future success.
However, I understand that when one desperately wants to believe something, then exceptions to the rule, rather the rule itself, can be framed as representing the greater probability.
What do they say? A drowning man will clutch at a straw?