Sheet films and rollfilms have a different base, which is of different thickness. That's why sheets and rolls do not come rom the same film stock. I think the boxes say what the thickness is on each type.

But I have an ancillary question: not all sheet films, as an example, have bases of uniform thickness, or so it seems. Bergger, for instance, feels siginificantly thinner than HP5, when you hold a single sheet. I wonder if this is a question of hardener in the base, or of thickness. And if it's of thickness, I wonder if that would affect depth of field at very wide apertures or very wide angles?