Hopkirk, the problem with having targets is meeting them. You want to lock up 100 burglars this year? If there are 500 burglars, you're failing. If there are only 50, you have to lock up 50 innocent people to meet your target. You might think targets for illegal immigrants are easier to meet, since there are (allegedly) so many of them. None the less, they've been deporting legal ones in pursuit of their target, and that's what caused the scandal.
wbm, I don't think she was misled by staff. She said there were no targets. A letter turned up in which she herself was discussing them with May. She must have "forgotten".