how the order promotion is applied?

Hi Team,

I have created order promotion(spend 500 get 5% off), item promotion(buy 20 item get 28% off) and both promotions are applied to one order.

case1: order promotion as a high priority.

I got the promotion price using below method 

        private Money GetDiscountedPrice(ILineItem lineItem, Currency currency)
        {
            return lineItem.GetDiscountedPrice(currency);
        }

But line-item promotion price has some variations.

case2: Item promotion as a high priority.

example: The order total value is the same for 2cases but item promotion value unexpected value in case1.

           unitPrice           qty   promotionPrice      OrderDiscount      SubTotal

case1:    $24.14            21       $372.09              $25.35               $346.74

case2:     $24.14            21        $365.00             $18.25              $346.74

my doubts:

Case 2: i am very clear in that. first, 28% applied then 5% applied.

case1: Expected is first 5% applied next 28% applied but I am doubted on the case2 promotion price calculation. Please anyone help me out,

how the promotion is calculated in the case1?

Advance Thanks,