Dell discount

Purchased a XPS 9560 from the outlet store for £1133.74 including VAT (£876.22 exc VAT). Also got additional 4 year premium support warrant which was priced at £257.52 making the total value of the laptop including VAT £1360,48. Dell had a 20% off all XPS models in the outlet store when I purchased so I made sure I used the discount code when I added the laptop to my cart. The prices quoted above are all before I used the discount coupon.

When I looked at my invoice after purchase it didn't show the 20% discount subtracted from the total price. Bit weird I thought so I phoned customer services and they confirmed to me that although it wasn't showing on my invoice, the 20% discount had been applied. Now I might be going full idiot mode here and if I am I apologise, but I just don't get how the 20% discount has been applied, if at all. Surely it's £1133.74 minis 20% which comes to £906.99 plus the £257.52 for extra warranty and the total should be £1164.51?

Here is what the bottom of my invoice looks like: