Unknown Device value. Difference between API and Value Tracks

Hi

Ive noticed in the Adwords API documentation that the field Device for the campaign-performance-report, has 3 options. The usual computer, Mobile, Tablets AND a 4th one called Unknown. But when you see the options in the {device} Value Track you only have 3 options,  "m" for mobile (including WAP), "t" for tablet, and "c" for computer. The unknown clicks reported on the campaign-performance-report  what value have assigned on the {device} valuetrack?

Thanks

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