There was no good comeback

This it the first summer where I am working full time.  Having a child who is a driver, while increasing my gray hair, is also increasing the freedom I have to be gone.  We have hired Sarah Katherine to be our nanny/driver/errand runner etc.  Every morning before I leave for work--while they are still sleeping and will be for the next 4 or 5 hours, I leave a very detailed, and yes color coded note, on everyone's responsibilities for the day.  I am very careful to make sure they know what time they need to be at practices, games, camps etc.  Overall, this has been working very well.  Occasionally I have to "show" them the proper way to say  fold a shirt--seems they believe that if it is just smaller and can be shoved in their drawers it's folded.  But again, overall, I have been pleased.  They are learning responsibility and are enjoying some of their new found freedom.
Last week, Chris and I met to take care of some banking business.  We were close to home, so we decided to go home for lunch.  As usual, I had left a note for the two children in town.  The note was surprisingly short as they had been so good at being on top of things.  Christopher didn't have basketball until 4, so for the most part, minus the few chores I left (one each), they were free.
Chris and I walked in the house (1:00 pm) to find both still in their sleep attire and on various technological apparatuses.  Sarah Katherine had already completed her one chore of folding the laundry.  I certainly did not want to compare them, but I decided to ask anyway why Christopher hadn't finished his--
Me: Christopher, why haven't you cut the grass yet?

Christopher:  (barely looking up), Because I didn't think you'd be home yet.

And with that, I went to make my lunch.

PS--the grass was cut by the end of the day with no more reminders.