Okay, I think I just figured this out. Seems I've been duped by an optical illusion.
The name of the game is shadows. I just realized in all the photos where you can clearly see the alternating rib sizes, the shadows being cast by those ribs are very helpful in recognizing the pattern. In both the "family photo" I posted earlier and John Korling's photo of the observation car, the lighting of those photos is such that the ribs cast alternating sized shadows. The effect is similar to how Bob highlighted the ribbing on his set by spraying it with primer then wiping it off. Now in my photos, the cars were always under harsh fluorescent lighting and so the shadows were nowhere near as noticeable. Looking closer at my outdoor photo and also shining a light down on the cars, I can now see the alternating large and small shadows that help to reveal the correct alternating rib sizes on the GGD cars.
So in the end it's a false alarm and another lesson in the tricks that lighting can play on your eyes. But I'm happy to have confirmed now there's nothing wrong with the GGD cars, and another accolade for that set in accuracy.
Alan, here's a funny thing for you, and this goes along with the whole optical illusion thing. Remember how you said the K-Line ribbing looked more like the prototype in the "family photo"? In reality, those cars only have half the number of ribs! I just counted, and there are 11 ribs running through the marquee board on the prototype. There are also 11 on the GGD cars, but only 6 ribs running through the marquee board on the K-Line cars. Yet they still manage to fool the eye into thinking they look more like the prototype. You were right, this stuff can drive you crazy!