The Happiness Index: Top and Bottom's Rule!

The Economist's widely quotes index, at least in the Australian press, shows a curious fact: the happiest countries are all either at the top of the world or the bottom of the world.

At the Top:       Canada and Sweden
At the Bottom:  Australia and New Zealand

What does that mean?

Being further away from the centre means you get less spin, perhaps?

And yes I know Chile is down there with the 'Bottom's'. But hey, that's the exception the proves the rule, right?