There are two main forces at work here. Your "position" relative to the person whose understanding you're correcting and that person's mental/psychological processes involved in the correction itself.
A few things go on inside that person. One relates to that person's "intelligence", whatever that really means or is (some good documentaries and lectures on this topic), which is usually a sensitive part of their self image. Another is their view of the world, how they interpret what they observe in light of their understanding of things.
So basically, when you correct someone's understanding of something, you first shock their system by forcing them to re-evaluate their view of the world and then their intelligence is insulted. Insulted in the worst way, you're telling them they've been conned into believing something untrue or didn't pick up on the joke.
Then their reaction will depend a lot on who they perceive you to be relative to them. Are you a threat? Are you a subordinate? Are you a superior? Are you a new friend? Are you a stranger? Or any number of the classifications we apply to people in our lives. And finally, once they're alone back at their desk or in the car, they start to wonder about all the people who they've discussed this with and who may not have been so bold as to correct them and just silently think they're stupid. And that's where the resentment begins.
Some people are more sensitive to this than others. Two categories being very intelligent people without a lot of formal education and rather dim people with a degree or position where people expect them to be highly intelligent. Some people are less sensitive to being corrected, scientists, for example who understand that their world view can dramatically change overnight (although PhD's tend to be pretty touchy).
Also, you need to check your own motivations and whether you need to correct these people or you just like to correct them. Sometimes the saavy move is to keep your mouth shut and let somebody else do the correcting. That takes a different kind of intelligence ...