Time - Federation takes time. Especially if servers are overloaded or haven’t allocated enough federation workers. Both of these things have been true at some point for most instances since the population boom.
Defederation - Different instances can defederate from other different instances. So say for example I have three instances, lemmy.one, lem.two, and kbin.three. In this example, lemmy.one has defederated from kbin.three and the post in question is on lem.two/c/community.
You now have three scenarios:
A user visits lem.two/c/community and sees all the content.
A user visits kbin.three/m/[email protected] (or however kbin does cross instance URIs, I’m not familiar). This user sees most of the content. The limited factor here being time.
A user visits lemmy.one/c/[email protected]. This user sees most of the content, but no comments from kbin.three because of defederation.
And I could be wrong, but I believe vote federation is separate from comment federation, so if that takes more time, then the comments sections might look different even if they have the exact same comments because of vote sorting.