I'm glad I'm not an Economist

Last week at Somerfield I saw a sale on 6-packs of Coke. The regular price is £2.19 and the advertised sale price is "3 for £3.50". I did the math in my head and decided someone had made a typo, since the sale implied that the 3rd 6-pack was not only free, but taking it actually refunded you 88p on the 2nd one.

Yesterday it was still there and since I drink a lot of Coke anyway, I loaded up with 3 rigs. Lo and behold the sign actually was correct. Somebody tell me, does this sale make any sense? I know they like to encourage volume with these schemes, but why not make it buy-2-get-one-free instead of making 3 of the item less expensive than 2 of same?

Plus, don't even get me started that one-way flights are twice as expensive as getting a ticket on the same exact airplane with a return at some random future date.