It's unlikely that clay-barring and waxing the car reduced its aerodynamic drag at all, as has been empirically demonstrated in wind tunnel tests. The explanation lies in boundary layer theory: the air molecules at the car's surface
do not move in relation to the car. Aerodynamics textbooks represent this phenomenon with boundary layer velocity graphs like the one below:
The air velocity transitions between wall velocity and freestream velocity within the boundary layer, whose thickness depends on the fluid viscosity and body length among other things. To actually affect airflow, you need a fairly significant protrusion into the boundary layer; a little dirt on the car's surface isn't nearly enough to do this. You can safely chalk up your mileage increase to your grill block and good driving.