From what I’ve come to understand, which I am no expert, I think it means that the posts from those magazines start when someone on kbin subbed to that particular magazine. I had found a pretty small mag on Lemmy that only had maybe 5 posts when going to the original instance but after I subbed to it I would see nothing on kbin. They made 1 post after I subbed to it and that is the only post to show up on kbin, even to today even though it’s been a week or so since I subbed to it.
I had read somewhere on here that when it comes to the individual communities, kbin will only federate with that magazine after one person on here subs to it, after that anyone in kbin will see posts from that day forward.
Again, I could be very wrong, I’m just guessing based on my experience.
TLDR: the magazine only shows posts from when someone subbed forward so it will always be incomplete, I think.