Light about to be shed!
To be honest, we never really talked too much about what kind of pistol the first weapon is. It's supposed to feel generic and slightly underpowered to make you want to buy something better!
It is indeed labeled as USP but we don't use USP-sounds, since the USP sounds (and is!) powerful and deadly. I think it uses the sound effect of a P228 pistol, which (at least in Counter-strike) has 13 rounds, but the number 12 feels much cleaner than 13.
Consider it some cheap low-caliber gun that sealers use to euthanize wounded seals.