Andy Burnham is a man whose policies and opinions seem to owe more to political expediency than they do to belief. He bangs on to the point of tedium about his own class, background and interests. As a result he’s widely seen as an unprincipled flip-flopper.

Sadiq Khan is a man whose policies and opinions seem to owe more to political expediency than they do to belief. He bangs on to the point of tedium about his own class, background and interests. As a result he’s the hugely popular mayor of London, the voice of those who’d be proud to think of themselves as the metropolitan liberal elite, and is even talked of as a possible future leader of the Labour party.

Oh, and also they were both born in 1970. So that’s a thing they have in common, too.

Why it is this approach to politics should have worked so much better for the mayor of London than the would-be mayor of Manchester is something I’ve been trying to work out for a while. There are definite parallels between Burnham’s attempts to present himself as a normal northern bloke who likes normal things like football, and Sadiq’s endless reminders that he’s a sarf London geezer whose dad drove a bus. They’ve both become punchlines; but one of these jokes, it feels to me, is told with a lot more affection than the other.

And yes, Burnham apparent tendency to switch sides, on everything from NHS privatisation to the 2015 welfare vote to the leadership of Jeremy Corbyn, has given him a reputation for slipperiness. But Sadiq’s core campaign pledge was to freeze London transport fares; everyone said it was nonsense, and true to form it was, and you’d be hard pressed to find an observer who thought this an atypical lapse on the mayor’s part. (Khan, too, has switched sides on the matter of Jeremy Corbyn.)

And yet, he seems to get away with this, in a way that Burnham doesn’t. His low-level duplicity is factored in, and it’s hard to judge him for it because, well, it’s just what he’s like, isn’t it? For a long time, the Tory leadership’s line on London’s last mayor was “Boris is Boris”, meaning, look, we don’t trust him either, but what you gonna do? Well: Sadiq is Sadiq.

Even the names we refer to them by suggest that one of these two guys is viewed very differently from the other. I’ve instinctively slipped into referring to the mayor of London by his first name: he’s always Sadiq, not Khan, just as his predecessors were Boris and Ken. But, despite Eoin Clarke’s brief attempt to promote his 2015 leadership campaign with a twitter feed called “Labour Andy”, Burnham is still Burnham: formal, not familiar.

I’ve a few theories to explain all this, though I’ve no idea which is correct. For a while I’ve assumed it’s about sincerity. When Sadiq Khan mentions his dad’s bus for the 257th time in a day, he does it with a wink to the audience, making a crack about the fact he won’t stop going on about it. That way, the message gets through to the punters at home who are only half listening, but the bored lobby hacks who’ve heard this routine two dozen times before feel they’re in the joke.

Burnham, it seems to me, lacks this lightness of touch: when he won’t stop banging on about the fact he grew up in the north, it feels uncomfortably like he means it. And to take yourself seriously in politics is sometimes to invite others to make jokes at your expense.

Then again, perhaps the problem is that Burnham isn’t quite sincere enough. Sadiq Khan genuinely is the son of a bus-driving immigrant: he may keep going on about it, but it is at least true. Burnham’s “just a northern lad” narrative is true, too, but excludes some crucial facts: that he went to Cambridge, and was working in Parliament aged 24. Perhaps that shouldn’t change how we interpret his story; but I fear, nonetheless, it does.

Maybe that’s not it, though: maybe I’m just another London media snob. Because Burnham did grow up at the disadvantaged end of the country, a region where, for too many people, chasing opportunities means leaving. The idea London is a city where the son of a bus driver can become mayor flatters our metropolitan self-image; the idea that a northerner who wants to build a career in politics has to head south at the earliest opportunity does the opposite.

So if we roll our eyes when Burnham talks about the north, perhaps that reflects badly on us, not him: the opposite of northern chippiness is southern snobbery.

There’s one last possibility for why we may rate Sadiq Khan more highly than Andy Burnham: Sadiq Khan won. We can titter a little at the jokes and the fibs but he is, nonetheless, mayor of London. Andy Burnham is just the bloke who lost two Labour leadership campaigns.

At least – for now. In six weeks time, he’s highly likely to the first mayor of Greater Manchester. Slipperiness is not the worst quality in a mayor; and so much of the job will be about banging the drum for the city, and the region, that Burnham’s tendency to wear his northernness on his sleeve will be a positive boon.

Sadiq Khan’s stature has grown because the fact he became London’s mayor seems to say something, about the kind of city London is and the kind we want it to be. Perhaps, after May, Andy Burnham can do the same for the north – and the north can do the same for Andy Burnham.