We have multiple factors, and your keyword is chosen based on these simple stats:
1) Keyword Relevance
3) Search Volume
4) Organic Competition (if requested)
5) Cost Per Click
6) CPC Competition
How a best "one" keyword is chosen factors all these into account while keeping the most competitive keyword at the top as your primary focus for the website. Therefore, sometimes these terms will be a much higher Organic Competition that any of the other keywords.