Having searched through grocery panniers, I would assume it's because that's how wide a grocery bag is. The Banjo Brothers Market Pannier (which I own) according to online specs is 8" wide, and it's the only pannier I could find in person that fit a Dignornio frozen pizza box in it. I see the Bushwacker is 9", which looks like it's just to made sure it's long enough to fit a grocery bag into.

I read that the Inertia Super Metro is 15" wide.