Take a look at the following two charts. These compare the price premium Apple charges for Retina Display when you keep everything else constant (including 8GB RAM).
With 15-inch MacBook Pro you pay $200 less to get Retina Display, keeping everything else the same.
With 13-inch MacBook Pro you pay $200 more to get Retina Display, keeping everything else the same.
If you know the answer engrave it on back of iPad min taped to back of 13-inch MacBook Pro and send to me.
Hint: it is in how much value Apple believes customer assigns to flash and its size that is causing this distortion.