وكذا يجوز أن يكون لأحد ملكة يقتدر بها على التعبير عن المقصود بلفظ فصيح من غير مطابقة لمقتضى الحال
Similarly, someone may have a faculty for expressing an intended meaning in fasih words without making that expression correspond to the occasion.
The passage concerns a distinction between capacities, not merely between one successful and one unsuccessful sentence. Someone may speak fluently about any subject yet fail to choose the right degree of detail or tone for the listeners.
Thus training in correct, clear expression and training in appropriate rhetorical choice are connected but distinct educational objectives.