Shouldn't 1 at pixel ratio of 2 be 4 pixels?
Since it's being doubled both vertically and horizontally, the 1x1 pixel becomes a 2x2 square, aka 4 pixels.
Think of the device pixel ratio as a linear measurement. One direction only.
1 css pixel equates to 2 horizontal physical pixels or 2 vertical physical pixels.
See if this older discussion helps you out: https://teamtreehouse.com/forum/device-pixel-density-of-2-means-4-device-pixels-per-logical-pixel