[lammps-users] Lost atoms with some scale values in the lattice command

Dear LAMMPS Users,
The number of atoms changes with the same input except different scales(denoted as a) in the lattice command.

dimension 3
units metal
boundary s s p
atom_style atomic
lattice fcc $a origin 0 0 0 orient x 1 1 -2 orient y 1 1 1 orient z 1 -1 0
region box block 0 3 0 1 0 3
create_box 1 box
create_atoms 1 box

if a=1,2,3,...or 3.625,3.635,3.675, the number is 192;
if a=3.615(lattice constant for Cu),3.665, the number is 190;
if a=3.605,3.645,3.655,3.695, the number is 191.

why are some atoms lost for some scale values?
Thank you for your time. Any help is appreciated.

Zhenhai Xu