They're results, not skillsets. Too many factors go into those results to get a conclusive evaluation. For instance, how many times was Howard targeted downfield compared to Njoku? How was he used in the redzone...as a receiver, blocker, the primary option, and so forth? And yards after contact is a very inconsistent measure since not all contact is the same, nor are the situations in which a player gets the ball. Not to mention the role the offensive scheme and surrounding talent plays. Heck, if all we had to do was look at the numbers, there wouldn't be any need for the combine, all-star games, pro days, private workouts, etc. Teams could just pull out the stat sheet and make their selections. No, teams look for the skillsets a player possesses to determine how they translate to the NFL. Stats are only an indicator to use as part of the puzzle.
So, just looking at the actual skills, what does Njoku possess that is superior to Howard? Has he shown better hands? Has he shown he can run better routes? Is he a better athlete? Is he a better blocker? The answer is no to all of them. Does that mean Njoku is a bad player? Absolutely not. But Howard clearly has the better tools, and is considered the better prospect by virtually every evaluator for a reason.