Sometimes there are material-specific moods which ask specifically for materials (I've seen dwarves have moods which specifically ask for adamantine wafers rather than metal bars, confirmed through DFHack.) This might be one of those cases.
This - I haven't fully analyzed the Strange Mood logic in any of the 3D versions, but it's been observed in the past that moody metalworkers can specifically demand adamantine wafers instead of "any metal bars" or even the metal bars in their preferences, though the circumstance under which this happens has not yet been documented. This shouldn't happen for generic metal bar requirements for other moods, though.
[edit]
I've just gone over the entire Strange Mood function in version 0.34.11, and here's how it works:
If you have any adamantine wafers available in your fortress at the time the dwarf enters a Metalsmithing mood, then that mood will
require adamantine wafers as its primary component. If not, then the game will look at the dwarf's preferences to find what their favorite metal is, and if you've ever smelted any bars of that metal, then the mood will require that type of metal as its primary component. Otherwise, any metal will do.
Moody clothers are similar - if you have adamantine cloth available, they'll insist on using it, otherwise they'll pick either plant fiber cloth, silk cloth, or yarn cloth based on their preferences (if they like any specific type of cloth, then they'll demand any cloth of that generic type - a dwarf who like GCS silk will demand any silk cloth) or randomly select one of the three.