till price higher than marked price

I went to focus to buy mitchum antiperspirant deodorant, it was marked to be R99, but the till price was R120. I refused to pay R120. The teller told me they do not have rights to change prices. To correct this she gave me R20. Why is the shop allowed to break rules by not giving supervisors powers and advise them to change prices in such instances? This is not the first time to experience this in this mall. The shops seem to be doing as they please.