It sounds like you are referring the federation. When something is posted to one server, that server has to then propagate it to the other servers. That process is not instant; especially in large servers. In fact, right now with the Lemmy software it is a bit of a bottleneck that will be worked on soon. issue/3230

Another possible reason for this is caching. Browsers and servers save pages a lot to reduce load and make things go faster. That means that I sometimes you can see an old version of a page for a bit until it refreshes. Set correctly, a cache shouldn’t get too stale, and using the browser refresh button could fix it.