You took some time to read the page and while reading it, the homepage changed and some new posts got added to the top. Therefore some posts get moved to the 2nd page.

I see that on Hacker News a decent amount as well when going through the top stories across multiple pages. My assumption has always been that the order changes between when I load the page and when I move to the next one (which sometimes is not for another several minutes).

The problem with Reddit is that the sorting isn't by time, so the content order changes.

The only solution I can think of for that is to track which individual posts have been shown to which users, which is quite a lot of work to do.