There have been plenty of reasons that pollsters have put forward for the discrepancy:
- The shy Tory - those that refuse to admit they vote Tory
- The lazy Labour - those Labour people who can't be bothered to vote
- The late swing - a last minute change of heart
- The non-representative sample - that pollsters tend to sample people who give them time (and there are more Tory professionals who don't have time to do polls)
The fact is that ever since 1992 the Tories have always been understated by roughly 3%, and Labour have been overstated in every general election by 2% apart from 2010. If you factor that into the poll figures then we get roughly near the final result. For that reason I think I'm more inclined to believe that the fourth explanation is closer to the truth and its a systemic problem.
Until the pollsters are proved to a general election spot on I'm going to be mentally adding 3% to the Tories and taking 2% off Labour in every poll. I suggest you do too.