Why do they do it?
Because of what happens when you tell the truth.
Let me to take you back to a Guardian account of the Hartlepool by-election of 2004 and what happened to the Liberal Democrat candidate Jody Dunn:
On August 27, Dunn had written in her blog about a dispiriting evening out canvassing with Simon Hughes. "It didn't just rain last night, it poured," she wrote. "In fact the evening became one of the more farcical moments of the campaign. We'd picked what appeared at first to be a fairly standard row of houses. As time went on however, we began to realise that everyone we met was either drunk, flanked by an angry dog or undressed."We have all had evenings of canvassing like that. But the account goes on:
The blog had continued with a joke about how Dunn looked like Worzel Gummidge in the rain. Ed Fordham had checked the copy as usual before posting it online. Nothing he read had sounded alarm bells.
The Labour printing machines turned again, and this time Hartlepool woke up to the news on its doormat that Dunn had accused them all of being "either drunk, flanked by an angry dog, or undressed".And given the opportunity, other parties would no doubt behave just as Labour did.
So it's much safer always to say you have received a great reception on the doorstep than tell the truth.