When the Tolerance Analysis returns the nominal value, it is building a default merit function for RMS wavefront error and returning the merit function value.

The Merit Function considers all fields and wavelengths in the system, but the Wavefront Map only allows you to look at a single field and wavelength at a time.

So usually, the Wavefront Map will give you a smaller value for RMS wavefront than the default merit function – the only time they will be similar if you have 1 field/wavelength in your system.