Two of the same tops in difference colours but only one is on sale

I went to the naked sale of 25 percent last week thursday, and i found two of the exact same tops design is the exact same, and also the same designer designs them, i took a maroon colour and a black colour in these tops, that I thought were on sale, and when I was paying at the till, the staff that was assisting me told me that the black top was not on sale, and i showed him that the maroon top was on sale, and the black top is exactly the same as the maroon, and i showed him that it was even the same designer, and he said he does not know the designers tell them which ones go on sale, i feel that this is not fair, how can you have two of the exact same tops in different colours and only the one colour is on sale and not the other, and I even found both the tops in the sale side, as a frequent buy im very disatisfied