For example, when Gallup comes across undecided voters, it pushes them to declare a preference anyway. Some polls have shown far less volatility. Last week, while Gallup reported the race veering dramatically from Gore to Bush, the bipartisan Battleground Poll reported a small but unchanging Bush lead. There are other differences. Certain voters, notably younger ones, are notorious for telling pollsters they plan to vote - when history shows they don't.
Timing is another issue.
Some pollsters poll on weekends. Others shun it. They fear they're less likely to reach married voters with children, skewing the results. People are harder to reach and less likely to answer telephone surveys. That makes it more difficult and costly for pollsters to achieve a representative sample.