I do not understand either why in some calibers the Partition weights are so close-look at 243 for example. In 7mm same thing.
I think it must be that the SAAMI twist rate for a particular caliber (like maybe a 257 Roberts) won't stabilize a 120 grain bullet but in the 25/06 it will. So actually, the 120 grain was never intended for the Roberts. The best example is the 22-250 twist rate of 1:14 will not stabilize a bullet much above 60 grains. But there are tons of bullets up to 80 grains in 224 caliber, what gives? Bolt action 223's and 1:8 twist rates is what. Its why you handload-to match a very specific bullet to your rifle.
Or, it could be that say Federal wants a Premium line of Partition bullets with a weight no one else has so they jack the weight around a few grains and that's the solution.