The student doesn't understand the general result, so asks for an explanation in a simple special case. The teacher fully understands the general result, and s

The student doesn't understand the general result, so asks for an explanation in a simple special case. The teacher fully understands the general result, and sees through to the special case like light goes through glass. So he just says yes, that's the special case, and sees no need to elaborate, because establishing the correspondence of the general to the specific is too obvious to bother putting in to words.