My guess is that they are thinking their experience is mostly a sort of standard one, with this window of visual experience, and some accurately represented sounds, and some reasonable thoughts about the things going on in their lives, and so on. But I guess that actually the same visual scene looks in some sense very different to different people, because of things like where their attention goes, what abstractions they use to think about it, and what associations and emotional flavor things have for them.
I think you are overestimating the amount of variation in experience that can be generated by focusing on different things in the scene or even having vastly different personal histories. The thought that your experiences are mostly standard is a rational one. Most people have typical experiences. I don’t think anyone would argue that our experiences aren’t colored by our unique traits but I think care needs to be taken not to romanticize and inflate the idea of “hugely varying experiences.”