If people aren’t stupid, why do they have so much trouble understanding me?

From Alfred’s journal

Spent most of the morning on the low branch over the Kleins' yard, telling Switch about the business with the crow and the hubcap, and he kept nodding in the wrong places. I said it again, slower. He nodded in the wrong places again, slower. For a good while there I held the comfortable position that the fault lay in his ears; if people aren't stupid, why do they have so much trouble understanding me? Then he told it back to me the way he had heard it and I found I had left out the part where the crow was already holding something. I had told the story forty times and never once told that part, because I have always known it, and knowing a thing feels remarkably like having said it.