Sometimes I think it’s just boredom. They’re getting a conversation out of it at least.
One of the other major reasons is that the truth conflicts with their perspective on reality. My ex was convinced that everyone in social housing gets given free furniture (in the the UK). Wrong, could be demonstrated in various ways online and also just by me being a social housing tenant and, well, knowing more about this than she did. Her worldview was that social housing tenants are the lucky ones and get given everything.
It took her a lot of work and experience living with me to make herself accept the truth.
She also thought social housing tenants didn’t pay utility bills, which sort of had some basis in fact, because there are some estates where some utilities are built in with the rent as part of the service charge, so you pay them all in one go - they are still listed separately on your bill, but they’re a flat rate. (It’s a terrible system that’s not very common and is being phased out). However, for people on benefits, or those who own a former social housing or council flat, those costs are separated and they have to pay them themselves.
That was much easier for her to accept because I could agree that she had a reason to think what she did. Accepting that she had simply got something “wrong” was much more difficult.
And occasionally if you realise you were wrong and they were right they will not let it lie, and you end up defending a position you don’t even believe in any more because they add little bits that you actually were right about, and there’s no polite way to end the topic.
With the bread/salt thing, it might be correct in the US but not the UK due to either common levels of salt in bread, or differing RDAs for salt. I know relativism can be annoying, but sometimes it can explain misunderstandings.
The problem with that particular nitpick is that it’s not hugely important that said slice contains 7% of your RDA, because it’s your overall salt intake that counts most. And generally that becomes high due to eating lots of products with too much added salt, not just a slice or two of bread a day. We do need salt, after all.