Advertised interest rates on new cars.

If I can explain per example. a dealer has a promotion on a new car and advertise it at 5% interest rate with a monthly payment of R4000 pm.  But when you want to buy it you are told your monthly payment can exceed R5000 per month. That is before service fees etc. Why is that? What must you do to get the car at the advertised interest rate and  monthly payments? Why are they allowed to advertise what  they are not going to offer their clients? I am ready to sign on the dotted line, but not for R1000 plus extra per month.