This is a form of over politeness that English people sometimes fall into. It's not common, but you do sometimes hear it. People who use this phasing do so in the (false) presumption of a rejection, as not to proposition the person being asked. So the rejection of the offer would be a positive response 'Yes that is correct', as they are being asked if they DON'T want something. This construction of a question this way would normally be rhetorical, it's a form of politeness not a real question, as the presumption is that can or will do what is being requested, but you are both pretending due to cultural politeness that this is not actually the case. eg. Speaking to a stranger - "excuse me, you couldn't pass me that menu over, please? The alternative eg. Speaking to a stranger - "excuse me, could pass me that menu over, please? Could possibly be seen as rather presumptuous, and rude. (How dare they ask me to do something? How Rude). Where as in the first instance we are assuming they can't and it's their choice to fulfil the request. It's stupid, and rhetorical - but English people are like that sometimes.