I let the cat out of the bag with my reply right below the original one. It's a mathematical idiosyncrosy. With that set of numbers and doing that math, in that order, you'll always end up with #9.
I had a math teacher in high school that liked to do stuff like that with his classes. He had a book with all these problems in them and he'd drive us nuts with that stuff. One day, one of Waupan's future residents stole the book and hid it in his locker. (that wasn't me, btw.) Poor teacher had a breakdown.
Anyone for a Weenie Roast?