Have you checked that both the Vac system filters are clean?
Your point is valid of course. But many pilots fail to write up perceived defects for fear of retribution from the operator or owner. This cavalier attitude is common. Not all operator/owners are kind and understanding. Thus the original question aimed at giving pilots something to hang their hat on in terms of suction indicated readings at selected rpm. The C172 Information manual does not make that clear and it is left to individual opinion as to what constitutes a defective suction pump or gauge.