How can I explain why my test gives me a satisfying shadow?

I hit 0.07 density 2 1/3 stop below 18%. That's a pretty satisfactory low density.

That corresponds to about 0.0079 MCS, this leaves room for a possible 0.0034 MCS flare.

I must assume that the difference between what you would expect from a 3.6% reflectance comes from effective underexposure due to a variety of possible causes; error in metering, incident meter not calibrated to 18% or maybe mechanical shutter speed problem. But it sure looks like I underexposed the test.

Which effectively gave me a good shadow placement.