PnP Promised me 20% but I only got 10%

Pick n Pay are celebrating there 10 yrs and I got a few specials based on being a member since then. One was 20% off any 2 bottles of whiskey. Redeemable at all PnP stores. So I use the opportunity to buy 2 bottle and only get 10% off. After arguing with the cashier, she says I must contact the smart card people. So on Monday I email ***. Today, Thursday, I get the following response:

"***meedah Davids (Pick n Pay) 7 Oct 2021, 17:01 GMT+2 Hello Jack,

Thank you for reaching out to the Smart Shopper team. We would like to sincerely apologize for the delay in our response, we are experiencing high volumes and currently doing our best to respond as quickly as possible.

I am sorry to hear about the experience that you've had with the Smart Shopper discount on your Whiskeys.

Kindly note that you were charged correctly. Each bottle was discounted 10% which adds up to 20% discount as a whole.

Should you have any further questions or queries please feel free to give us a call on the number below, alternatively chat to us on WhatsApp, see how to add us as a contact below.

Call us on: ***or email us on: *** Operating Times: Monday - Saturday 08:00-17:00 / Sun & public holidays: 09:00-17:00

Kind Regards, ***meedah"

As a percentage is a ratio of the total 10% of half plus 10% of another half is still 10% of the total. It does not become 20%