Sport coupe/Cabriolet rear window

Does anyone know why the Cabriolet uses a smaller rear window frame than the Sport Coupe? The Sport Coupe window is 9 5/8 x 20 1/2 and the Cabriolet window is 8 3/8 x 19 3/8; same as the Roadster and Phaeton.
Thank you in advance for your thoughts!
Don
 
It had a lot to do with seeing behind with the rear view mirror. The Cabby has a short windshield support
frame and different rear view mirror.
 
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