All paints specify ranges for offsets and rotations. On application, random values are chosen within those ranges so that each application is unique.
Paints also specify a range for wear. On application, a random value is chosen within that range. As the wear increases, more scratches appear on the surface and the weapon becomes grimier.
The texture responsible for the grime and the one for the scratch pattern are both placed randomly so that each wear pattern is unique, even when the wear amount is identical.