A lot of the new rounds are short action Vs long action guns.
It helps alot when mounting scopes, and when reloading. Less powder for more velocity. It may only be 100FPS, but the pressures make a difference as well. We are in an era when the old powders we used have been reformulated for higher performance, and higher velocities with less barrel pressures.
In the past, SAAMMI used a copper crush to develop the max loads for center fire guns. Took awhile to get the data from that method.
The pressures were expressed in CUPS(copper units of pressure). Most people that reloaded or just common shooters didn't understand nor fathom the importance of pressure in the chamber until a gun exploded.
Now SAAMMI measures in PSI as they have load cells that can measure the pressure instantly on a computer, vs the Copper Crush method that took ten round or more to establish an average.
Pressures are set by SAMMI depending on the guns on the market currently, and those that have been on the mkt in the past.
For instance, the 30-06 has a SAAMMI max pressure of 60,000psi. The .270 has a max pressure of 65,000psi.
Reason why? Early models of the 06 had weak chambers and lots of them are still in existence, the .270 is a modern cartridge, and was designed for these pressures.
Same as the 45-70. Lots of the Black Powder versions in rolling block are still used to this day for deer rifles. SAAMMI will not allow manufacturers to make anything more powerful than the BP rifles can handle.