Think of it like a book. It's going to the table of contents and saying these pages are empty and can be used. Yes words are still on the pages but now you are free to erase what's there and write something new. This is why things can be recovered.

Destructive delete is deleting the table of contents entry then going to pages and erasing everything on those pages. Some methods will then write over the data with garbage several times. So with the same analogy it's erasing the page writing gibberish then erasing it so even the intends of the last letters are impossible to make out.