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
I think it is for mobile phones as well see https://github.com/piwik/piwik/issues/3416
Eg in case of iPhone 5 it would be more accurate to log
1136 x 640 because from a website point of view it is only