Burger king making up different prices at different resturants

I went to burger king in century city about 4 days ago to get 2 burger specials (2 for R100) and when i got there i found out that the price had change to R110... but thats not the issue... the problem was that yesterday i went to the burger king drive Thru at Zevenwagcht mall... and there the same meal was R115... so my question is are that just making up theses prices... how do you rob people like this