I bet it was regularly "serviced" by a dealership whose sole goal was to get the vehicle in and out of the service bay as quickly and profitably as possible. It cuts into the profits to take extra time to replace the #11 & 12 plugs, and if the dodgy dealership is lucky, maybe they'll fail outright and the vehicle owner will bring the car back to to be repaired. Hooray! More profit! Cha-ching!