I used to piss people off by claiming to be a vegetarian when I was pescetarian.
I genuinely thought that I was saving everyone who might host me trouble by saying 'I'll have the veggie option, I'm vegetarian', because nobody would want to be arsed listening to/catering for 'well I do eat fish, but not meat', & I was always going to be very happy with a vegetarian meal.
It was politely pointed out to me by an actual vegetarian friend that this was in fact irritating as fuck, because occasionally in our friendship circle she'd have to explain that no, she did not eat fish, despite the fact that 'oh but Corvus says she is a veggie, & she was scoffing a tuna sandwich last time I saw her, so vegetarians do sometimes eat fish, right?'. Fair point.
I imagine it's similarly annoying for vegans to have part time/not quite vegans muddying the waters by sometimes eating things with dairy or egg in them.
I do make an effort to cook vegan at least a couple of times a week, because it's a great way to try to get as much veg as possible into the family, but I'm definitely not vegan. Couldn't possibly get past the horrible cheese substitutes, tbh.