It records res = 2560x1600 because window.devicePixelRatio = 2 and actual resolution is 1280x800. It should actually report 1280x800 because it does not show 2560x600, it only shows more pixels "per pixel". Similar issue might be for smartphones but haven't tested.

Related code: https://github.com/piwik/piwik/blob/2.16.1/js/piwik.js#L5022-L5024