That white pouch is an inherent feature of all bow/xbow-armed units with Mamluk-style heavy mail.

As such, Moorish Crossbow Militia and Granadine Crossbow Cavalry with heavy mail upgrade have this pouch.

If you're going to give MA more ammo, you're going to have to do it for these units as well.

Here's another interesting tidbit:

Dismounted Arab Cavalry have sword scabbards but they can only wield spears. This is because they use the same skin as Arab Cavalry which can actually switch to swords.

Small mistake on CA's part.