But it is also - perhaps primarily - the result of who they know.
As Andrew Neill, the personification of that oxymoron, the working-class Tory, and ex-employee of the Murdoch press, showed in his BBC2 piece last night, politics on both sides of the divide is today based on a series of cabals.
As we know, one of these cabals met for dinner in Chipping Norton over Christmas. The Camerons dined with James Murdoch, the Europe and Asia Chairman of News Corps, and Rebekah Brooks, the Chief Executive of News International.
They did not, of course, discuss the Murdoch empire at all.
They didn't discuss the phone-hacking scandal at the News of the World, nor the responsibility of Cameron side-kick Andy Coulson for these crimes.
And they certainly didn't discuss the desire of Murdoch to buy the remaining 61% of shares in BSkyB, and whether such a move should be referred to the the Competition Commission.
Why should they? It was only a couple of days before that Cameron had removed responsibility for the referral from Vince Cable. It wouldn't even have been at the back of their minds.
So what did they talk about?
Cameron's favourite joke? They had probably heard the one about Clegg before.
Institutional sexism in football and Sky Sports? Perhaps. After all, the presence of Mrs Jeremy Clarkson might have been to ensure a modicum of political incorrectedness in this studiously politically correct gathering at which nothing of any real import was discussed at all, if we are to believe the prime minister.
Maybe they really did sit in silence, appreciating the food and wine their hostess had laid before them. That way there would have been no breach of the ministerial code, and nothing to connect them with those who would stitch up the country for their own profit.
Today's listening: Little Walter Jacobs. A collection of recordings featuring the guy who played harmonica (and occasionally guitar) with Muddy Waters and other greats, compiled by a friend. Thanks Tom.