The "6" comes from the infamous "666" spoken of in the Book of Revelations. Take that away and what do you have 9.11 --- the day that none of us will ever forget.
It all makes perfect sense to me... and don't forget. Brush and floss at least 3 times a day and see your dentist at least twice a year.
I'm not a dentist on a message board, but I play one in real life.