How did this result happen? My post-vote survey

I surveyed over 14,000 people on election day who had already cast their vote to help understand how this unexpected result came about. My poll came very close to reflecting the outcome of the election, with 41% saying they had voted Conservative, 39% Labour, and 9% voted Liberal Democrat (the result in Great Britain has been 43% Conservative, 41% Labour, 7% Lib Dems). The survey found two thirds of those aged 18 to 24 saying they voted Labour, as did more than half of those aged 25 to 34.