Probably their parents are Catholic, and they go home and go to church on Easter and Christmas. They don't care about memorizing the ceremony or all the rules, because honestly it's pretty boring, and let's be honest if they really believed they would go every Sunday and practice. "God is good, and Mom wants me to go" is good enough.

My parents made me go to church every Sunday, CCD class every week, was an altar boy, did all the sacraments. Up until Confirmation, which they told me to make my own choice - and I did. They never brought it up again, or nagged me to go to church. They still say grace at holiday dinners, but are cool with us not doing the song and dance and just bowing our heads respectfully.