Back up to retail price?

Me and my dad where going to buy a toyota scion tc (07). It was listed in our local newspaper for 13,888. They said I could get 4,000 off for the trade in of my 1996 toyota camry. They did some paper work and we left. After getting home and actually looking over it, it says the 4,000 would be off the retail price of the car. Which was 15,000. WHY? If it's listed in the paper for 13,888 why are they going back up to retail price then??? When it comes down to it, we really aren't saving at all then!?