Leather Cuirass Lorica Musculata, I used to think no way but

[Tarbicus]

I'm not throwing the baby out with the bathwater. If it's not leather in the representations it sure ain't metal. So what could it be?

Either the sculptor is using artistic licence, or the item being depicted is costume and not armour.

Bingo. I'd go for the latter if I had to go for something. Still could be worn by an officer but not as something that would stop an arrow or weapon. For show. Part of the package, not functional. Behind the lines stuff, or even for civic functions.