Why his teacher seemed confused ?

Robby has a tendency to forget to tell me things, like the fact that he has a choral concert coming up. Being a pro at being the Mom of children in the mandatory public school music programs, I know that every December there is the beloved ( cough cough dreaded cough cough)concert. Since we are well into the season I asked Robby last night when it was. He thought for a minute then told me the sixteenth. OK ,I said as I started to explain that I need for him to let me know of these things as soon as he finds out as we are an extremely busy family. "Don't worry Mom", he said. "I already told the teacher that I wouldn't be there if that is the night that You, Dad & Cindy go to Prison" "Oh" I said with a smile.... "Did you tell the teacher that we were going to the prison with a church group to sing Christmas carols ?" "No, she didn't ask and besides, when I told her she looked at me kind of funny and then went and sat down before I could say anymore." Oh I said..... I wonder why ???