Could you interpret these screen resolutions from Google Analytics?
1440x900 is the native resolution for 15'' non Macbook Pros, and the given  (best/default) resolution for 15'' rMBP with the pixel doubling effect taken into account, as it is. The analytics don't distinguish between the two, so the most you can infer from the data is that the most common model is a 15'' Macbook Pro.