Why do some millennials think it's an insult

My mother (born 1985) absolutely denies that she is a millennial whenever I've brought it up. She gets offended, as if I've insulted her? And I know several other people around her age who also feel similarly. Why is this? I'm a baby (core z, '04) but was very sheltered+poor growing up so I didn't have a lot of the typical "gen z" experiences with early internet/technology so for a long time I related more to millennial experiences (to clarify, now I do think I am 100% gen z I'm talking about my experiences and opinions as a young child)

Tldr: I don't get why certain millennials get offended by being called "a millennial"