I don't see any argument for having your saddle low as a beginner or any other time. It only creates knee strain. My experience is that people come in with it low, not high.
Fitter B might be measuring to some point on the saddle nose. That would explain the disparity. Most fitters measured to the sit bone contact point. But Retul measures to center. The amount of disparity between the two numbers and the degrees of knee bend stated suggest a larger change than just between sit bone and center, though.