You are correct, Catholics are a subset of Christianity... But similarly how people assume a "doctor" is a medical practitioner, Christians has become the informal name for "Protestant" or "evangelicals"

Basically "Christians" tend to mean, anything not "Catholic" (which is old school, visibly indistinguible from others in the Christendom)

Probably because depending on the context "Christians" is likely referring to "Protestants." There are some very significant differences between Catholic and Protestant Christianity, moreso than between Protestant denominations, whose differences tend to be a bit more trivial. Other comments make some good points, but it is not too far of a stretch to say that Catholicism may be different enough to be considered a separate religion (I don't know who gets to draw these lines). But in the most technical sense, yes, Catholics are a subset of Christians.

They mean Catholics and Protestants but they're morons and their religious leaders have convinced them that Catholics are somehow not Christians.

LDS people will also consider themselves christians but protestants dont like that lol