My guess (and it is only that) as to why the trigger plate bluing flakes more or quicker than the receiver itself is that the trigger plate has a different heat treating done to it.
I think the difference in heat treatment/hardness effects the way the earlier blue of the pre war guns adheres/wears on those parts.

From the way the trigger plate cuts compared to the frame itself, I'd guess the trigger plate had little or no additional heat treatment hardness given to it at all.
The original mfr frames can be anything from very tough to near impossible to cut. Very tough on tools, even carbide.
The trigger plates cut much easier.