As chambering reamers are used, they require re-sharpening. Sometimes the last few rifles are cut with very small chambers as a result of reamers being sharpened one to many times.
OK - there are minimun and maximum tolerances from reamer produces, rifle makers, SAAMI and die manufacturers. At times a few tousanths of difference can cause problems. This is what happens.

If you have reloading DIES cut with a NEW reamer (long die chamber) and a rifle CHAMBER cut an OLD one (short rifle chamber), the die might not be able to shove the case shoulder back far enough to allow the case to chamber in the rifle. It happens more often than one would believe.

Shaving a tich off the bottom of the dies, will allow them to shove the shoudler back far enough to allow the dies to size the case to fit the rifle's chamber. It's an easy fix and one I've done several times. No big deal.

The big problem comes when a guy buys short dies and has a long chamber. He then can push the shoulder back far enough to creat excessive headspace. This happens mostly with old green box dies.