Shell Petrol Station Rossouw street,(Pay for 29L ON 25L Jerry Can.)

I visited Shell Rossouw Street Motors Murryfield Pretoria with a 25 L Jerrycan for Generator. I use 95 Unleaded octane petrol for the Genirator. After filling the 25 L Jerryb Can the Pump reading shows 29.62 L with an amount of R707.90 to be paid. I Questioned the Leters reading on the pump and my Questions could not be answerd. I tryed to call Owner Sibisiso on Cell No. *** with no result. My Question is how do you get 29.62 L eters of Petrol in 25 Liter Jerry Can.