Agreed. It is very odd. Especially as the tang of the trigger guard is exactly where one would expect heavy wear from rubbing by dirty fingers. Unlike the magazine plate, which is hardly touched by the hand in normal use, but in this case is corroded, even slightly pitted.
So I don't think the plating was applied overall, and then wore off in this somewhat unlikely fashion. Instead, I rather suspect that Bubba partially plated the badly corroded areas after buffing them. A fudge, in other words.
If there is a more plausible explanation, someone please enlighten me!