Because it takes into account those moments you spent at the signals (0kms/hr) and your accelerations from gear 1 and slow moving in busy traffic and so on. It's the gross average, inclusive of slowest drive in city traffic also.
Come on, I hope you didn't believe Volverine bro cruising at below 40 and spent a day and a half reaching Peshawar and back again, hahahaha!
Regards.