I often experience this. I saw that a co worker had written something about God in their Twitter bio. "Are you a Christian?" "I'm a Catholic" they replied. Any other denomination would say "yes I'm a Christian" (there are no denominations in heaven, nor was there in the early church).

To understand this behaviour, it should be known that Catholics have introduced man made rules that they have additionally decided are not up for discussion (infallible) even if the Bible appears to say otherwise. Catholics teach that there is no salvation outside the church. By definition, this makes sense - the church is by definition a body of people who belong to Christ.
However what the catholics actually mean is "the Roman Catholic church". Whether your average Catholic realises this or not it's debatable, but the common clarification "I'm a Catholic" is because they have absorbed a corrupt teaching that only catholics can be saved.
Read this and the linked article at the bottom https://www.catholic.com/magazine/online-edition/is-there-re...