Example: my mother watches tv with me, the character might say, the dog needs surgery because it broke its leg. Within seconds she say, why are they going to operate on the dog? This happens constantly. I am constantly repeating within seconds what what just said on a program we are watching. The other thing she does is ask me for example, is that her husband, what is going to happen, where is the dad, do they have a mom? Why is she doing this? Is this dementia?
Not being able to follow a story line ( the dog has an injury, it must need some sort of intervention, surgery is a possibility) is pretty typical of dementia patients.
As Glad says, it could also be a hearing problem. If she is concentrating hard to hear some of what is being said she may be missing other things. Have you tried watching things with closed captioning?
Best thing to do with Mom's remarks is to agree with her, than trying to correct her. That way it would be more of a win-win, then generating a long going disagreement. You won't want Mom to be afraid if someone is telling her something she said is wrong.