This may not be stupid, but it's "arithmetically declined."
My wife used to work for a landscaping company. Employees got a 25% discount on plants. One day, the place was having a 25% off sale, so my wife picked out a bunch of plants and went to the register. The check-our girl said, "OK, you get 25% off for the sale and a 25% employee discount, so that's 50% off."
My lovely wife, being the honest person she is, explained that that's not how it works.