Why are communities on Lemmy.ml more full compared to the same on a different Lemmy instance?

https://lemmy.sdf.org/post/13415

Why are communities on Lemmy.ml more full compared to the same on a different Lemmy instance? - SDF Chatter

For example, I’m on Lemmy.sdf.org [http://Lemmy.sdf.org] and I joined the [email protected] [[email protected]] community. But there are many missing articles and comments. If I browse directly on Lemmy.ml [http://Lemmy.ml] for the same article there ~90% of comments are missing. https://lemmy.ml/post/1152794 [https://lemmy.ml/post/1152794] has 26 comments https://lemmy.sdf.org/post/11232 [https://lemmy.sdf.org/post/11232] has 0 comments

This could mean that syncing is in progress between these two instances. In that case, if you check back in a bit, you should see all the comments.

I think there might be more going on as the post in question is 4 days old, and all the comments are at least 1 day old; is the syncing typically days behind? The federated version that OP links shows 0 comments, and the version on my instance has just 5 comments.

Is it to do with when a user on the remote instance first interacted with the post? I.e, its only showing comments from after someone on lemmy.sdf.org first interacted with the post?