Assuming it works the same way on Kbin as it does on Lemmy, and that you’re talking about your All feed rather than just your subscriptions:
You’ll be seeing posts from the communities and magazines your instance already knows about. Which, at least on Lemmy, means at least one person from your instance has subscribed to them (it may or may not be a different metric on Kbin, not sure).
Given that it’s easier to search for Kbin magazines on Kbin, I expect there are a lot of things people on your instance sub to that people on lemmy.world do not, and vice versa. Hence, different feeds.
There’ll also be differences in the algorithms for what counts as “active” or “hot” or whatever on each software.