When going through the post feed, I'll randomly get 5-10 new posts added to the top of the feed. These 5-10 posts are all new with 1 or 2 upvotes only. Sometimes all from the same community.
This happens on local, subscribed, or all feeds. It also doesn't matter what sorting option is selected.
It's a known bug. If you're using Firefox on desktop like I am, going to about:config and setting "accessibility.blockautorefresh" to "true" seems to be a workaround.