Statistically, he may have put up the best PPG for his career in one season, but "best" season doesn't mean highest PPG. There are many, many other ways to determine abest year as a pro.
KG is a perfect example. I think 2008 was his best year as a pro, even if statistically it wasn't. I would guess KG would say the exact same thing.
Hayward can have his best year still in him without scoring 20 PPG, especially if his excellent play leads to deep post season success.