Apologies, I was thinking about this the wrong way.. you are perfectly correct, the nominal cartridge length is actually the chamber length, which is the critical dimension.
I suppose the point I was trying to make is that case length for roll and star crimp is different. The roll take less case to form and the assembled cart is longer if you use a crimped case. Normally you would make up the difference with wads etc or you make up a shorter length assembled cart if you star crimp..