Anecdotally, many people get a bachelor's degree to check a box for job applications, whereas many people get a master's degree because they love the field and/or are afraid to leave school.

My friends and I who have a bachelor's degree in CS make more money than my friends who have or are working towards master's degrees in CS, because the former are working in the private sector and the latter are in academia making peanuts.

Other possible reason could be many or most Masters degrees not conferring additional pricing power, and those people’s Bachelors degrees also confer lower pricing power.

Edit: Another possible reason that Masters degrees were less common in the past, so the Bachelors pay statistics skew towards people with more work experience in their higher earning years, whereas the Masters pay statistics skew towards younger people with less work experience.

Masters seems to be a common theme in a few lower paying expansive fields like social work and education. I don't think that someone with a masters is typically making less in the same field all else equal.