I’d also say it reflects WHERE you live. California, The Mid-Atlantic, Great Lakes and southern New England states, where so many Catholics live shun guns.
In the post war period the gates were opened to Eastern European Catholic farmers ~ Ukrainians, Russians, Poles, more Germans, etc. The railroads made it possible to convert the vast American desert into a world of wheat and other grains. Coal mining brought in even more of them.
Once all the plants were planted, fields ploughed and barns built, the Italians and Greeks, all Catholic, came into the Eastern cities ~ and California ~ to build urban infrastructure.
Consider the large urban areas that have for so long had gun laws that forbid gun ownership and right there you have a notworthy percentage of the Catholics who a survey like this would show as not owning any guns. It’s probably more a matter of their not admitting they own a gun.
In my experience, a lot of people in a lot of places that have laws against owning guns have at least a pistol and/or a shotgun they don’t admit they have to anyone they don’t know well. Years ago I knew a guy in NYC that traveled the Mid Atlantic states as far south as VA and who sold guns out of his basement to people he knew well and who said they wanted a gun.
I’m sure he sold a few dozen guns to various folks during the years I knew him, either to people he met and got to know or who his wife met and got to know. People were paranoid abut more riots and more roving bands of thugs so they were glad to have the guy get them one and make a few bucks in the middle. None of those people would never admit they owned a gun to some random questioner and given the social functions this guy was always a part of his customers were probably almost all Catholics.