In my opinion, this is just common usage of English words, specifically as related here to automotive engineering.
So sure, in automotive engineering, a "cylinder" is part of the engine block, while a "barrel" is a sometimes-cylindrical vessel, inside an old-fashioned carburetor. But by the way, the four barrels of Rochester carburetors are not necessarily cylindrical. The secondaries look more like rectangular prisms.
But in other fields, a "barrel" would be a cylinder-like object, with bulging sides. Such as a wine barrel. It's not a cylinder, in the mathematical sense.
I wouldn't make too big a deal about this. I think it's just a linguistic convention used, applied to internal combustion engines, in your case.