Ummm...isn't that the point of using an average weight? If 50% are over the average, the other 50% can only be less than the average. So all is OK.
Just sayin'.
The real problem is that frequently it's not 50% who are fatties and over the assumed average weight, it's 80%, especially when they lug on board 25 Kg of "cabin baggage" that no aircraft is designed to accommodate.