Poor advertising

Today I went to food *****s Dobsonville mall, they have advertised that you can "FILL A PACK" onions for R20 so I filled my pack and the cashier said I must only put 13 onions not filling a pack but I told them it says fill a pack and cashier told me the manager told them not to allow more than 13 onions. I am so disappointed because when I go to other food *****s and they say fill a pack I fill it without any problems. Maybe I don't understand a sentence fill a pack. Please inform management to advertise correctly because you cannot say fill a pack and at the same time have conditions, and why other food *****s allow fill a pack and Dobsonville doesn't allow that?