As mentioned previously, it's not the case length. Also, it is not critical that a die is wrench tightened in the press, only tight enough that it doesn't change during reloading and finger tight can assure that. The radius of the bullet's ogives can vary a little bit, but not by some 25 thousandths.
It's much more likely it's your press linkage is varying its springing a little under the varying seating pressure of using mixed brass. Seating pressure is quite light anyway and there is, of course, a layer of oil in the toggle linkages and pins. More of that lube will stay in place if the case necks are soft or thin, so the ram will raise higher and the resulting OAL will be slightly shorter. On the other hand, when the case brass is harder, the seating pressure will be higher so more lube will be pressed out of the toggle links; meaning it's all compressed a little more. The OAL in that case will tend to be slightly longer because the ram won't rise up quite as high. It's all just the mechanics of presses.
And, as alluded to in a post above, the amount of spring is a little greater with turret presses. That compounds the problem.
