How can it be?

Writers' characters are real. Ask any of us; we'll tell you.
But sometimes it's scary how real.

How can my characters realize something I don't, and tell me about it? One of the first times this happened was during a conversation I was writing between two characters and the protag told the other character something and as I wrote it, I thought, "Now wait. Is that true?"
I checked and yes, he was correct. But how did he know that when I didn't?