Because its units were not perm assigned, but assigned depends on the division's needs. CC-A and CC-B's only real unit is the 3-5 light tanks and 65-99 men(depends on pre-1943 or later). CC-R is the reserve, which worn out units within the division were rotated into CC-R for R&R and rebuild of the unit. CC-R was changed to CC-C in 1954.
CC-A, CC-B, and CC-R/C term is changed/redesignated by US Army to Brigade in 1960s.