Actually the behaviour seems not to be consitent with the spec. To quote the spec:
"For a CSS device, these dimensions are either anchored (i) by relating the physical units to their physical measurements, or (ii) by relating the pixel unit to the reference pixel. For print media and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit."
So for example a 3rd generation Apple iPad with so called "Retina display" has a screen resolution of 263.92 PPI which is much closer to a 300 DPI print than to the usual 90 DPI of a computer screen. Google Nexus 10 features 300.24 PPI. So I would say these devices are what the spec call "print media and similiar high-resolution devices" and 1px sould be 0.75pt or 0.26458333mm.
Using 2x2 device pixels seems to be just as wrong as using 1x1. 2 display pixels on the Nexus 10 for example are just 0.1692mm wide compared to ~ 0.26mm width of the reference pixel the CSS spec uses.