Different comment counts between instances is a federation thing, yes. But showing a number of comments different to the comments shown on the same instance, not.

There are reasons for missing comments, especially on communities from lemmy.world right now. But that's not what is being reported. A federation message for a post does not contain the number of comments (and that would be a totally pointless property to send). It's also quite common that certain comments don't make it over federation.

The screenshot is showing a comment count without comments, that means the instance has one comment locally but for some reason is now showing it. That's not a federation thing.