If you simulate only the asymmetric unit, then CHARMM computes that volume, and not the volume of the unit cell.
It should be noted that the value used for CRYSTAL BUILD CUTOFF can (and probably should) be larger than the list cutoffs CUTNB and CUTIM. For CRYSTAL BUILD, the value determines the list of available crystal transformations, and too small a value might leave out needed transformations.
I always preferred using the full unit cell for small molecule crystals, and sometimes multiple unit cells, esp. for force field tests. How much the molecules deviate from each other when more of them are real and not images can be useful information.