I believe it&;s because the discount card changes the price total so the 15% buff is calculated off what the new &quot;total&quot; is after the discount cards. It&;s still stacking the discounts as you&;re still getting the 15% off on top of the 20% off it&;s just not adding them together like 35% total.</p>
<p>If you think about it, it&;s very similar to on-sale items in stores with coupons, the items are on sale at 20% off and without coupons you would get them at 20% off the original price, but with coupons you can get an additional 15% off, that doesn&;t make the item 35% off the original. So the 15% is like a coupon being added to an already base discount/sale price.</p>
<p>Hopefully I&;ve managed to explain this in a way that makes sense.</p>
<p>In addition, not sure this is a problem or bug, this might be more suited for the site feedback forum?