The order in which a discount is calculated is important.
In the Discounts FastTab on the Retail Setup page, you can specify where in the calculation sequence the coupons are calculated. If the Coupon Discount Sequence is A, the coupon discount % is applied to the original item price. If the Coupon Discount Sequence is G, the coupon discount % is applied to the price of the item after all the discounts with a sequence from A to F have been calculated.
For Sum all discounts are applied at the original price.
For Line the highest discount is applied to the original sum, the second highest to the price that is then available, and so on.
For more information on how the priorities are calculated see How to: Work with Price Priority.