Feynman's story here was that at dances/social events, when he said he was a professor, and that he had worked on the Manhatten Project, he was assumed to be ly

Feynman's story here was that at dances/social events, when he said he was a professor, and that he had worked on the Manhatten Project, he was assumed to be lying. So he switched to avoiding saying those things, and implying he was a mature student instead (plausible since the war had delayed the education of many people).