قَصْرُ قَلْبٍ إِذَا اعْتَقَدَ الْعَكْسَ
Qalb is used when the listener believes the reverse of the intended judgment.
Suppose someone holds that Khalid is a poet rather than an orator, whereas the speaker intends to establish the opposite in that comparison. مَا خَالِدٌ إِلَّا خَطِيبٌ then does more than add a fact; it reverses the hearer's erroneous attribution.
In ifrad the hearer combined the alternatives. In qalb the hearer confidently assigned the wrong one. Distinguishing them requires attention to the listener’s belief.