I'm assuming (although nobody's actually come out and said it yet), that it's similar in intent to the 'are you intending to bring down the Government of the US?' question that they ask on US visas. It's not that they're expecting the paedophiles to list every e-mail address and chat nickname that they might potentially use, it's that if they find a paedophile using a nickname and/or e-mail address that they haven't declared, they can arrest them for that.

I think that at least makes sense; I'm not yet sure whether or not I agree with it (and I recognise the potential for misuse of such powers is huge).