This is my take on the matter as someone who learned many of the hymns by ear long before ever seeing the musical notation as a choir member.
Many of us Catholics learned to sing hymns without accompaniment. We may have had the words but many of the old missals and prayer books did not have the notes. When you don’t have notes then hymns are more likely to take on a more chant-like style which is driven by “feel”.
“Feel” in terms of singing is usually driven by the way the community “has always done things”. Some communities are filled with trained singers. Others are filled with those who listened to recordings of hymns that might have been “stylized” by the singer. I would also note that people from different ethnic backgrounds and different regions have varied speech styles which can impact singing. In other words, not only do people speak with different accents but they also sing with different “accents”.
Have you ever noticed how the meter of a poem seemed off to you because it had too few or to many syllables? It was probably because the author of the poem ran some syllables together or added extra extra emphasis to syllables in a way that is different from your style of speaking. This frequently occurs in songs and hymns. --or at least in the way they are sung at Mass.
People have a natural sense that you should slow down at the end of a line, a verse, or a hymn. It’s a breathing based matter that chant incorporates. But without a leader most people have difficulty agreeing on how much to slow down. And they run out of breath because unless they are practiced singers they don’t have any sense of how much air they will need. A slowing tempo and lack of air often has problematic results when it comes to final notes.
In the case where I learn a new hymn in conjunction with an instrumental arrangement, I often find that when I later have to sing that hymn without accompaniment, I am mentally listening to the instrumental flourishes between the lines and/or verses. I think others do this too but they may not be be hearing the same arrangement I am. And then there is the added matter that when singing unaccompanied the interlude between lines or verses should be eliminated.
In my opinion the majority of Catholics do NOT read music. Even if the notes are provided most people are simply looking at the words. If it’s a hymn tune people have memorized they will sing using the memorized melody (which may have been a localized variation.)
All of these variables (and no doubt many more) go into the odd way people sing hymns.