On most unixes, you don't need to own a file to delete it. Instead, you need write permission on the containing directory (and if you don't have write permissions on the directory, you can't delete a file even if you own it)
That's not true for directories though. If a directory (c) has files in it, the owner of the containing directory (..) can't delete it because they can't (necessarily) delete the contents of the directory (c/*). And the owner of the directory (c) can't necessarily delete it unless they have write permission on parent (..).
I've only just noticed that difference in behaviour between files and directories. Its never been a problem. (of course, I have root on most systems where it would be so its easy to work around). So I guess this counts as obscure?