David Cameron and Lord Ashcroft's tax status

We are told that David Cameron found out only about a month ago that Lord Ashcroft was non-domiciled in the UK for tax purposes. Before then, it would seem to follow, he believed that Ashcroft paid tax in the UK on all his earnings.

So how do we explain what Cameron said in December of last year? Then he told the BBC's Politics Show:

Lord Ashcroft's tax status is a matter between him and the Inland Revenue. What I can say and what he has said is that the undertakings he gave at the time of being made a peer are undertakings that he is meeting.

But if Cameron believed Ashcroft was paying tax like everybody else in December, why didn't he simply say he was? This quotation reads like a carefully prepared form of words.

If David Cameron did not know Lord Ashcroft's true tax status in December, it sounds as though that was because he had been careful not to find it out.