Distribution of order discount does not take conditions of discount into equation

Everything I have read indicates the formulat for distributing the cart discount among the items of the order is:
To calculate the discount amount for each item, Magento uses the following equation:

$discountRate = Item price X Qty / SUM of all the items ordered

$finalDiscountAmt = Discount total X $discountRate

My question is: When the cart contains an item that is defined in the cart rules as excluded and therefore is not included in the coupon discount, why is this item given a portion of the discount?

Example: Cart rule is $10 off $39.99 purchase
Included item: $29.99
Excluded Item: $10.00
Included Item: $15.99

The discount calculations should not show a discount applied to the Excluded $10 item – but based on the formula used all three items will get an equal percent discount shown in the order details. How can we apply the exclusion rules of the cart discount to the discount distribution in the order details?