Mike,
I have no firm idea why Nosler does that, but it might be due to the longer case and overall cartridge length of the 7x57, which provides a little more room for powder. Though I have loaded 50 grains of H4350 with 140s in the 7mm-08, and while the powder charge was pretty compressed, it worked fine.
They also might have done that due to only having so much of certain Ramshot powders to play with.
