I don't get it

Today I had to get bloodwork done in the morning so I set up the lesson plans for the day for D to handle while I was gone. He had at most 2 hours alone with the boys to do work before having to pick me up. Half an hour before he even had to come get me, the boys had finished most of their work. I just don't get it. I usually spend all day on their case, harping on them, and we still are doing work until mid afternoon. How is it that the one day that D works with them, they are done so quickly??? I just don't get it.