This is a great question and certainly confusing for those not used to the federation aspect.
A quick explanation:
If the magazine has a domain name at the end of it that isn’t the same as your kbin account, then it means that the magazine’s “home” is on a different instance.
Example:
technology@fedia.io
That means that all the instances (servers) will push content and updates to the “home” instance and receive updates and content from the “home” instance.
When an instance (server) subscribes for the first time to a magazine on a different instance, it will start to receive new content from that moment forward. However, older content isn’t pushed out to the newly subscribing instance. You can think of it as subscribing to a newsletter or something. You won’t automatically be sent copies of all the older content but you will get new things moving forward.
You can have older content show up by manually entering the direct link to the older content into the search bar on your home instance (in this case kbin.social) but it’s a manual process.
That why the message shows up about “may not be complete” since it doesn’t know how much total content there is on the remote instance.
This topic (called “backfilling posts/contents”) is one that has been discussed on the Fediverse for some time.