When I sign in to Apple Bug Reporter (aka Radar), the sidebar shows these sections:
What's the difference between Closed and Archive?
I don't have any actual hard info here, but my belief is that Archive is just closed issues that haven't been touched in so long that their info is no longer cached and must be fetched from Radar again if accessed. If you actually click on an Archived issue, you'll find that it gets removed from the Archive, which is why I think it's just based on cached info.