I have two theories in place:

## 1) Embarrassment

One candidate is controversial / unpopular. Respondents are unlikely 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 leading side that may grow overconfident.

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

**EDIT / UPDATE:** I wrote about two matters that I've discussed with my wife. No sources, no references, just pure intentions and sharing our experiences. It could be improved... I could spent endless research, references, sources... I'm at the source, one of my domains is "mostly doing" because I've learnt that talking about stuff doesn't move the needle.