What is Mean Roof Height (MRH) and does WFCM impose a limit on it?

In the WFCM, Mean Roof Height is defined as "the distance from the average grade to the average roof elevation." See WFCM Figure 1.2.

The WFCM is limited to a Mean Roof Height (MRH) of 33' or less, but the envelope method in ASCE 7-10 that is used in the WFCM has a limit of 60'.