Better steel is the answer but....
If you want some fun read P O Ackley Handbook for Shooters and Reloaders Vol. I pg. 203: Barrel Steel.
Its easy to use terms which describe steel and hard to prove it is better.
Loading for the .30-30AI with both a top eject and Angle Eject rifles I know the Angle Eject rifle will accept higher pressure loads (based on a sample of 1 top eject and two Angle Eject rifles) but I cannot tell you why. I can show you writing on the internet and possibly in a gun magazine or two where the writers describe the Angle Eject Model as weaker due to the cut in the right receiver wall.

I was discussing this with a friend and found this in my folder on Winchester 1895 stuff. From the same American Rifleman issue, October 1945 a second opinion on Winchester Model 1895 strength.

[Linked Image from i.imgur.com]
All threads need a picture of an old hunter. Warren H Miller was a VERY interesting man.
[Linked Image from i.imgur.com]


Slim