I actually remember finding this one while working on Recettear and making funny, confused faces at it for a while, but it's not a bug. EGS clearly differentiated between 'metals' and 'precious metals'. Now, we could use 'base metals,' but not everything under 'metal' is clearly a 'base metal.' Especially IIRC some of the 'this comes from weird ruins!' stuff might have trouble deciding if it should be base or precious metals, and furthermore I'm not sure a lot of people are familiar with the term.
It's an oddity, I agree, though. I'm sorry you had trouble with it.