On deleting old content from a collection - (18 collections in all!). The collection still retains some of the data although there are no articles within it.
Might be worth looking at this? Image attached of a collection that is EMPTY apart from one banner - yet all these 'unknown' entities are still listed.
Turns out there is a big delay when deleting articles, and the system seems to remove them one by one.
Had to wait 30 minutes to see them all disappear.