Probably the thing about databases getting fragmented, so they delete large swaths of data and optimize all at once, rather than piecemeal when the users delete it. It's well known that when you "delete" a photo from Facebook, if you save the direct link to it first, you can still access it (go ahead, try it). But the one time I tried it, the photo did finally disappear after about a week (although I have seen some people claim that they could access deleted content via direct links for up to 6 months).
Probably the thing about databases getting fragmented, so they delete large swaths of data and optimize all at once, rather than piecemeal when the users delete it. It's well known that when you "delete" a photo from Facebook, if you save the direct link to it first, you can still access it (go ahead, try it). But the one time I tried it, the photo did finally disappear after about a week (although I have seen some people claim that they could access deleted content via direct links for up to 6 months).