One somewhat misconception is that all chrome plating is created equal. In some respects it is, in other respect, it's not.
The "bumper chrome" is harder and shinier, for example, that then interior brightwork, which has a more satin finish, by comparison. There are also different "shades" of chrome, too. I observed that factory bumpers and such on Toyota pickups (1990s) had a more blue tint to it, as USA models were more "white" or "silver". Little nuances like that can give vehicles a different look, by observation.
The less-hard the chrome might be, the more porous it can be, as a result. Which then means it does not protect as much against moisture intrusion into the lower layers and the base metal underneath. Which can result in the "rust" forming.
There are several DIY ways to remove rust deposits, including DIY solutions to apply to the surface, let it sit a while to chemically dissolve the rust, then wash off and neutralize before it is further polished/waxed for protection.
Consider, too, that if enough moisture collected on those particular items, it might also be on the underside of the roof areas, which did not receive any more than basic primer at the assy plant. So investigate further when you remove the parts for cleaning. Might need to apply some rust encapsulator products?
Enjoy!
CBODY67