In 2009, I knocked on a door in one of the leafier bits of a Labour seat. The man who answered was incredibly angry, stark naked and – I remember this clearly – greying, everywhere. He was not voting Labour: we were all – including me, he added – claiming expenses and giving his money to lazy immigrants. There was no chance, he said, of him voting Labour ever again.

Labour held the seat by 4581 votes. The naked man is neither representative of the seat nor of my experiences on the doorstep in the run-up to the 2010 election, but he is the only voter I really remember speaking to.

We call this the “availability bias”: we are more likely to remember colourful and striking encounters than routine ones. If one in every ten voters loudly hates the Labour party, the rest are polite but are cooking dinner/watching television and quickly tell you their vote – ie., they are a normal person, you may just as quickly forget them.

That may be why so many of the Labour activists I spoke to seemed to be so glum about the by-election, and why my impression of the seat was so negative.

My guess on Oldham West – in this week’s podcast – was that it would be a Labour hold of about 2,000 votes. They won by 10,722 votes. But at least I was in good company – most of the media, most party members and MPs, and both parties themselves privately thought there would be 2,000 votes in it.

The same constituency chairs who told me they were “confident” it would be Jeremy Corbyn who won the Labour leadership were equally convinced that the Oldham West by-election was going to be a close one. The same organisers who accurately predicted a mere one-point swing to Labour in the marginals, in defiance of the polls, were off-the-mark as far as Oldham were concerned. Ian Warren, one of the sharpest thinkers about political demography, predicted a split of Labour on 41 per cent to Ukip on 36 per cent, with turnout tanking.

The fear stretched across left and right of Labour. I talked to one former campaign staffer from the Corbyn camp who feared they were “triply fucked”: voters who wanted to hit Isis would leave, voters who were disappointed at airstrikes happening wouldn't turn out, and others would be turned off by the party's public squabbling. Ukip, too, were confident of getting within 2,000 votes of Labour.

It could simply be a few outspoken and angry former Labour voters will linger longer in the memory of most campaigners than the hundreds of positive responses.

Or it could be that Labour’s North West operation simply used its activists very well – if your activists are spending a lot of time talking to firm Labour promises before the final days, they may be missing out on persuadable voters.

It was the North West that saw some of Labour’s best results in May 2015: gaining Wirral West and City of Chester against the tide. It may be that the reason why so many Labour members left Oldham convinced it would be tricky is because the campaign team sent them to exactly where they wanted them to go, meaning that on the day itself, they could be confident of only talking to cast-iron Labour voters.

Ironically, the one source of information that got it about right was the polls. Labour got exactly the result we’d expect in a by-election from their current poll average of 30 per cent. Perhaps, having got the Labour leadership election right too, the pollsters should be let out of the doghouse, at least cautiously.