Hole Command, Tap Depth

Does anybody know how Pro figures the drill and tapped depth for a hole? I am adding a M16 tapped hole in a steel plate. A M16 x 1 gives a drill depth of 45 and tapped depth of 36, M16 x 1.5 gives a drill depth of 43.5 and tapped depth of 34.8 and a M16 x 2 gives a drill depth of 42 and tapped depth of 33.6.

Obviously there is some relation based on the pitch.

Using WF4

Thanks