Because ROC on Approach is not the GPA. Simplistically, it is 1 degree below the effective GPA, and begins at the threshold crossing at 200'ROC (yes, below ground)
The FAA model is very simple, it assumes a 3 degree GPA, a 200' ROC, originating at a straight line down at the threshold, sloping up at 2 degrees until it is a 500' ROC, that is the location of the FAF...
its just that simple...