Actually, that's not a "most of the time". It's required. According to page 2.82 of the Standards and Policies Manual:Most of the time Dive Masters crossing over to NAUI from other agencies are required to pass the NAUI Master Diver exam before entering into the Dive Master curriculum.
(The NAUI Assistant Instructor course has the same in its list of prerequisites, which is to be expected, since NAUI AI is a strict subset of NAUI DM.)Divers with evidence of equivalent training and experience may be enrolled provided they pass the NAUI Master and Rescue Scuba Diver written examination with minimum scores of at least 75% on each.
Of course, the 75% is only the minimum by standards. The instructor is free to require a higher score if he so chooses.