They definitely did not apply the same measurements of one bag for all sizes. Here are the specs of the bag's measurements directly from the website:
Dimensions: (Regular) 84"L x 33"W, (Long) 90"L x 33"W, (Short) 78"L x 33" W
Keep in mind we are talking about a 0 degree bag here, not a 3 season bag. It's likely that EB generously sized these bags because of their intended use...high altitude mountaineering most likely. In these situations you need extra length to stuff you boot liners, fuel, electronics and other widgets into the bottom of your bag, otherwise they freeze overnight. I admit, seems a even a bit long for that purpose, but it's still a possible explanation.
I do think that it's off that they all have the same down fill...I would venture to say that that info is incorrect.