There's always the possibility that some parts got mixed up or are wrong alloy/heat treatment. I'd be inclined to send it back if you or any future owner might some day shoot a single .357 load through it.
good observations, I thought about this same thing when I purchased the gun; I have the box with the "spent" .357 casing; I have shot .357 with it; I own more PD340's all nib and all including this one weigh exactly the same.