One of the things I learned was that polls tell you what they tell you - and that is all they tell you. Yesterday's poll about voting intentions doesn't tell you how people are going to vote next week. It merely tells you how a random sample of people questioned yesterday said they intended to vote. Within limits, it may help you to extrapolate possible outcomes. But at the end of the day, Dewey still did not defeat Truman.
Sam Clemons (aka Mark Twain) is attributed with the saying, "There are three types of lies: Lies; Damned lies; and Statistics." Personally, I don't think it's so much that statistics lie as that people assume statistics show things they simply don't.