each method you mention above in you question depend on certain assumptions and methodology...the AASHTO 1993 method is regression method based on pavement performance obtained from road test, whereas The backbone of
KENLAYER is the solution for an elastic multilayer system under different type of axle loading, so its considered mechanistic-empirical design method..from my experiences if the input data is defined well for the both method ,,the difference does not exceed in worst case 10 perecent..