If I've understood correctly the EB stat guys do not treat the "shield" value as something directly tied to the qualities of the defensive item in question. Instead, they factor in stuff like the soldiers' skill and technique of using the things and overall method of fighting, and then adjust all two or three component values accordingly. For example, AFAIK quite a few of assorted barbarian infantry with barely clotches on their back have a semi-decent "armour" value to represent them using their shields to block and deflect missiles etc., but nothing remarkable in the "shield" score. Conversely the assorted phalangites and suchlike are given relatively high "shield" values to make use of that score's "directional" nature in the game engine to represent the comparatively "linear" fighting style of such troops, and presumably it also interacts with the phalanx special formation in some fashion.

I think the Romans also fit into the category whose shields are partly counted into the "armour" and "skill" defense scores - that seems to be fairly typical of comparatively loose-order close-combat infantry. I would assume the purpose thereof is to represent the comparatively fluid and flexible, "all-around" combat ability of such troops who do not depend extensively on drill and formation for their defense.