what the...?

HB often wakes up from his nap with a messy diaper, or messes it promptly upon wakening. I was not surprised when I walked into his room this afternoon and smelled the smell.

I was, however, astonished that he had pooped entire logs ON HIS BEDSHEET, while he was wearing a diaper and a cover and overalls. All were still on him when I walked through his bedroom door. But the poop was in the bed.

So I ask, how is this at all possible?