R50 discount became R30

Had a birthday voucher that expires on the 31st, (R50 off), so I bought an item of the R150 and used the voucher at the till. Two ladies, ( helping each other) punching at the computer, came up with the answer that I am going to pay R120 for the item. I am still suprised: R150 - RR50= R120. I wonder what happened to the R20