When you pressurize the system from the MAF mount you're also pressurizing the crankcase because of the vents on the front of the valve covers. They hook in directly to the inlet side of the turbo and allow the air being used for leak testing to enter the crankcase. Also, there will be a set of intake valves open somewhere in the engine and some air will always get past the piston rings, pressurizing the crankcase. Neither should be an issue for leak testing.
