It can be, but more details would help. Some calibers have greater brass variations than others. It might also matter at what level (near min/near max) the powder charge is.
If you are using mid to high level powder charge and everything else being the same except brass, would agree with others that the brass thickness in the neck may be less. Some chambers will have get carbon on the necks simply because the chamber neck is not real tight, or you fired enough rounds to get it dirty.