I have two theories in place:

## 1) Embarassmnet

One candidate is controversial / unpopular. Respondents are unikely to admit to a surveyor about their true preference. At a voting station there is privacy and true preference will take place.

## 2) Determination

Supporters of the side that is slightly behind are more motivated to turnout and vote. As opposed to the winning side that gets lazy.

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I'm not a doctor, not a lawyer, not an investment advisors, just some theories, common sense and critical thinking.