This is kind of odd. It's recently been discovered that iOS has been holding on to deleted SMS and iMessage attachments. The software stores all attachments in the Library/SMS/Attachments directory, and when they're erased from Messages, they're supposed to be erased there too.
But as many folks have pointed out, that doesn't seem to be the case. One user in particular, on a quest to determine what the 'Other' category listed in his device storage consisted of, found that the directory contained thousands of sub-folders with images he was sure he had deleted...