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They bow out a bit. 12L will be an empirical measurement from stuffing it with things of known volume until it is almost bursting at the seams. How practical that would be on the road is another question.
You definitely won't get 12L of stuff in one of them and keep it at 6.5cm width. But it bulging out ~1.5cm each side won't get in the way of legs and will increase the volume significantly. A triangle that's 55x35x6.5cm is as underestimation too.
I did some calculations on the question above.
The 12 liter Apidura bag is a triangle of approximately 55x35cm and the width is 6,5cm.
So: (55x35x6,5cm)/2 = 6256 cm3 = 6,2 liter.
How can it be 12 liter?