It's more than that, though. A car is essentially an elevated trapezoid in cross-section, and you're sitting near the narrow top of the trapezoid. A Ford F-150 is essentially the same thing, but slightly less tapered and the base is lifted higher up. Either way, you're squeezed into a lot less than that 6.66' of width. Private automobile doors are also much thicker than bus doors, there's space allocated for airbags, there's stuff on the inside of the door that takes up some of the width.

A bus is essentially a rectangular prism projected all the way back. It uses the FULL 8'4, minus the width of the walls, for every row of seats. So it's probably more like 3-4' extra width compared to a car.