This is because Lemmy uses websockets to retrieve live data from the server. I imagine this was a good thing when Lemmy was unpopulated, but now it's just in the way. It's also causing strange bugs, like the vote count rapidly changing to wrong values, or a post just completely changing into a different one while you are reading it.
Websockets will be removed from Lemmy in 0.18, which should fix all of this.