I would GUESS that it has to do with how powder works. I don't have the equations of combustion handy but you can figure it out logically.
People tend to view a bullet going down a bore like a spitball through a straw. You blow on it with more or less constant pressure, accelerating the bullet down the bore.
The reality with combustible powders is different.
The rate of combustion of a powder is proportional to the pressure of gas around the grain. The higher the pressure of gas the faster that it burns or deflagrates.
If you just pour smokeless powder onto a flat surface and light it it fizzles slowly. Atmospheric pressure isn't much.
Put it inside of a pressure vessel, ignite it and the pressure rises to a point where the vessel ruptures or the powder is consumed. Whichever comes first.
The higher the pressure the faster the combustion.
So you have this closed container, where the powder burns at some rate because of pressure against it.
Pressure and volume are inversely proportional. Increase the volume of a gas and its pressure drops.
Think of the cartridge and barrel as a long tube. The powder is ignited by a primer. The pressure rises to the point that the bullet is pushed out of the cartridge into the barrel bore. The butt end of the bullet seals the tube at one end. The other end is sealed off by the cartridge.
Inside of this pressure vessel the powder burns, dependent upon the pressure.
A heavier bullet is "more resistant" to motion. Inertia. So the "seal" in the pressure vessel will move more slowly. Slower change in volume means that the peak pressure goes higher than with a lighter bullet.
Think of powder as the amount of energy you can safely put into the system. You have to go lighter with heavier bullets to keep from blowing it all up.
With a lighter bullet you get a faster change of volume. The lighter bullet has less resistance. So you can push the bullet with more energy.
The rate of change of the volume with a lighter bullet will be faster, all factors being equal (same powder, primer, case and diameter of bullet). This faster change in volume "burns" the powder slower because the rate of change of the volume is faster.
This is a lot easier to explain with some equations.... or some graphs.
The green is the internal pressure of the bore (it's a mythical average because the pressure will vary all through the combustion zone), the blue represents the velocity of the bullet in the bore, and the red its position.
If you use a heavier bullet that green curve will "rise" because the powder has to work harder and thus the gas pressure will rise to drive combustion to a higher rate. If you go too high the pressure vessel ruptures.
So those values are not "stuff" but more like a speed limit. It's more "what can I get away with before I rupture this thing?"