Barrel length is not a factor in peak pressure.
Are they using the same exact bullet, brass and primer?
Different brass has different powder capacity. Different bullets, even of the same weight, can be different length which can effect the remaining capacity inside the case.
Besides that, different lots of the same powder can vary by 10%.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.