There is one new factor this year, that the pollsters may not have taken into account.
There were heavy drops in registration this year, as individual voter registration started in Great Britain.
In Oxford, even after a big registration campaign, 6% fewer people were able to vote than last year. In student areas registrations were 30% down. And in areas where large numbers of people move out and in to private rented accomodation, some 20% weren't registered.
Unless the polling surveys ask people if they are registered to vote, they will not have taken account of a registration system designed to disenfranchise people who are not settled, long-term residents: the natural Conservative voters.