Due to inherent limitations of the various payment processors we support, and the need to round prices to the nearest whole-cent, sometimes the discount total calculated in a buyer's cart will necessarily be up to a few cents short of what you'd expect.
For instance, some processors cannot support a flat discount taken off the order total; in that case we'd have to split the discount amount across all items in the cart, reducing their individual prices when we send the order to checkout, and if that total discount amount isn't evenly divisible by the number of items, there would be a remainder or rounding error. The discount shown to buyers in their cart is the total discount they will actually get, so as not to be misleading.
