The axle rating is only one aspect in a whole lot of other calculations go into that final CCC rating (brakes, suspension, frame stiffness, etc.).
When I was looking for the truck with the highest cargo rating I was routinely disappointed in what I saw in the F450's glovebox.
I don't believe for a moment that any truck manufacturer is "downgrading" any ratings on their big trucks, they are always pushing the numbers higher to get bragging rights in that class.
The 450 is best for towing.
The 350 is best for cargo carrying.
But if you chose to "read between the lines" be prepared to live with the C$O$S$T of that decision.