It is due to the profilling that Polling co's now do on polls.. they were so far out in 2015 that they now model the polls.... it depends on what the model they have written says to what the outcome would be. E.G they think that more people who say they will vote Tory will actually go to the polling station and vote compared to the Labour voters.. the same with undecided, they model that of x% that say they don't know, more will end up voting Tory than Labour, so they stick that in the Computer and come up with their numbers