Don't expect stock even if it is advertised

I wanted to purchase the Xbox One on special from Incredible Connection in Cresta for R3999. It is clearly advertised on their shop windows and in the newspapers. When inquiring about the deal the sales assistant was unpatient and rude telling me that all of their branches across South Africa is sold out on that offer as well as the Xbox 360 Infinity bundle for R2500. How can you be out of stock of a product when it is valid from 16 to 24 December and I want to purchase it in 18 December.