Doors of store closed before closing time

On Friday, 13 February 2015 my mum and I got to the above mentioned store at approximately 18:45 and upon our arrival we found the doors closed with staff member Zimasa on the inside telling us that they are already cashed up! The sign in the window clearly states that the store closes at 19:00. My mum told them that its not 19:00 yet and we were then let into the store. While paying, my mum again asked the till operator why they close the doors before actual closing time and her response was \ Mam