"Dave" Cameron has decided that he's going to have an immigration quota, but he won't say how big - probably because he knows that any actual decision will be unpopular with either the Daily Mail ("too high") or business ("too low"), or - more likely - both.
He's also decided that he's going to have a points system for economic migrants. The problem is that you can't have both. What happens if too many people get enough points?
I suppose you could have a system where you grant visas to the 75,000 people with the most points - effectively like a norm-referenced exam (like the pre-1987 A level). If that is your system, then your points has to be meaningful at all levels, rather than just at the pass-fail level, which makes the scoring much more difficult to set up.
Of course, knowing what you're talking about is not a job requirement of either politicians or interviewers, so no-one has asked Cameron the key question.