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 more...