That is not unusual. The differences between 8mmI (.318") and 8mmS (.323") cartridges as well as minimum bore dimensions were only fixed by the 1940 proof law. As both the Austro-Hungarian army in WW1 and P.O.Ackley in the 1950s found out, it is not the too tight bore that causes excessive pressure, but the chamber and throat dimensions. A max S cartidge in a minimum I chamber has neck and bullet wedged into the neck and throat area of the barrel. On firing the case neck cannot expand and release the bullet and the bullet, wedged into the barrel throat, cannot move until pressure has risen to danger levels. Mauser was well aware of this fact. In Jon Speed's first book, "Mauser OOSR" on page 275 a hand written table from the factory about sporting rifles bore dimensions is shown, dated December 9, 1926. 8mm bore dimensions given here are clearly I, .318" ones. But on the bottom of this table there is a footnote. Translated into modern English parlance it reads:
"The sporting rifles for the 8x57 and 8x60 cartridges get the throat for the S bullets, so that the S-type (.323") cartridges may also be used."