If a regular FL die works in the OPs rifle, what benefit does he gain from the extra .001 sizing?
Just give it time. Someday in the future he will use range pickup brass or brass shot in one of his other rifles, or some loads were a little hot and he was unable to size them down, and then the problem will surface.
Might as well ask, how and why are factory cartridges the dimensions they are?
Might have something to do case clearance after firing.