BTW, there's quite a few differences between a duster and a trench coat.
Dusters are designed for riding horses, to help keep the dust stirred up by the horse off the rider's clothes.
What I imagine may have gone through the artists mind...
"Hmm, this summary the publisher sent for the art says he wears a duster. Isn't that like a trench coat? Doesn't say anything about a hat... but he's a wizard and wizards wear hats but he's a present day wizard so pointy is right out..."