Polling is hard; there are lots of ways for polls to be inaccurate (see the British Polling Council postmortem of the last general election for the gory details

Polling is hard; there are lots of ways for polls to be inaccurate (see the British Polling Council postmortem of the last general election for the gory details). Those errors don't add up to 20 points. And I say that as a card-carrying member of the Labour party.