Why do all metals corrode when attacking a Rust Monster, yet it itself can only target ferrous metals

When you attack a rust monster, the following may apply:

Rust Metal. Any nonmagical weapon made of metal that hits the rust monster corrodes. […]

So, bronze for example being a metal would corrode.

But the rust monster has the following action:

Antennae. The rust monster corrodes a nonmagical ferrous metal object it can see within 5 feet of it. […]

But seeing as bronze is not ferrous, it cannot be targeted.

What is the lore reason behind this?