Overcharged

I visited a Mr price store in Springfield Park Durban today which I purchased a top for R179.99, it didn't have a tag stating the price but when I got to the till that's what i was told to pay. It so happened that I ended up visiting another Mr price store in Musgrave centre and I came across the same item but it was being sold for R159.99 I've been buying from Mr price for years and I know that no matter which store you visit the prices are always the same. So I'd like to to know how I was overcharged for this item. Who knows how many items this could have happened in the past but I didn't notice. This is very wrong