Free wine glasses at Wilkinsons stores

Basically this is working the same as the free 16 piece dinner sets.
The glasses are marked at £10 for a pack of 4, or 2 packs for £15.
They are in the Xmas reduced 1/2 price sale,.
For some reason they go through the till at - £7.50 each set, then deduct discount of £7.50 + £7.50, making the 2 sets actually free!
The barcode on the 2 packs I just bought were : 0257 6398 & 0257 6381.
Clear glass with black and silver circles on stem and base.
There was also a red set that I price checked that worked the same.
I had to come home by bus so couldn't carry them today, will try tomorrow.
They had plenty in my local branch tonight.