Some measure the OAL of jacketed bullet rounds; they will have different OALs because the length of the bullets themselves vary. Cast bullet length variance will be less, maybe much less and this will be refected in overall cartridge length.

For reason or reasons unknown by me, different brass and brass of different lengths can cause OAL fluctuations, even wth cast bullets of the same batch. None of these variations are of any real significance for shooting purposes.

Significant variations in length are something entirely different and a serious problem. I don't believe that's what we've been discussing in this thread.