Because others are unreliable observers of ourselves, that's why.
"We agreed that he sounds a lot like you," she said.
I'd never heard that one before. But what next scaped her lips was even more surprising.
"We also think the two of you look a lot alike."
"But he has a tiny roundy head," I said. "And mine . . . ," I didn't have to finish the sentence, as it is neither tiny nor roundy.