Those chest pieces are historical. The archeological record and what artwork exists from that period tells us that's basically the kind of armor they wore, but they also probably wore some hellenic-style armor. I do see one guy in the first Italian Spearmen picture with a linothorax, and maybe there should be a few more with that kind of armor in the "Aristocratic" unit. Here's some historical evidence: part of the "sarcophagus of the Amazons," from Tarquinia, c. 325 BC (in the archeological museum in Florence). The picture isn't completely intact, but that armor looks like a linothorax to me.
As to them looking "lower-tier" than they are meant to, remember that these aren't professional soldiers, but citizens armed with whatever weapons they can buy. Only the wealthy soldiers could actually afford helmets and breastplates. For the Italians, this only really changed once the Romans switched to a military armed and paid for by the state (the Marian reforms).
But this mod does have some strange armor values. Looking at those Royal Spearmen, they do seem to have heavier armor than the Italian Aristocratic Infantry. I don't know how these armor values are decided; it may have something to do with balance, but these aren't the only units that have armor stats different to what the armor actually looks like on the unit. I think one example people often mention are the Greek swordsmen, who have a higher armor value than hoplites, while the unit models wear what appears to be less armor.