Because what you're compairing is not as straightforward as you'd like it to be; the total resistance of the material is based on the section, or, if you prefer, the thickness of the said material; remember when the Mythbusters made a cannon with duct tape? Just add layers one over the other and you get more strenght, hence more .built-in. safety.
That's why a M/98, even if it was made of the same 1020 steel, would always be stronger than a M/94/96/38. And that's also why Paul Mauser used a thicker receiver to suit the then new to come high pressure 8mm.
The material resistance is based on standardized tests so they can be compared one with the other.
You can find references of material analysis used by the Swedes from both P.O. Ackley and Jerry Kuhnnhausen. HVA also often bragged to have the purest, low carbon steel of the world.
Sir Charles Ross, when it was availlable, also prefered the Swedish pure, low carbon steel to make his receivers.
As for the reason why both the 9X57 and the 9.3X57 were/are kept at a low pressure, it seems that both rounds have always been problematic because of the thiny shoulder they share. If you carefully look, you will see that new the 9.3X57 cases often show a protruding primer after intial firing. After successive fireforming, the phenomenom disappears.
Then, most ammunition manufacturers rarely load their round at MAP pressure; they usually keep an average of 15% margin, for liability reasons.