The thing is that there are almost an infinite number of things you can learn and you only have enough time to learn a few of them. So if I talk to someone else, there is a very small chance that person spent their limited time learning the exact same things I did. Of course, this depends on context. At work I assume that the people who also work in the quite specific niche I’m in have a large overlap in knowledge, but a random person I meet anywhere else probably won’t even be aware that this thing I know a lot about is even a thing. Not because they are stupid but because they spent their time learning other stuff I know fuck all about.