Confused and Irritated

So I arranged for an Uber van to get me to a particular destination. With a promo code activated, the trip came to R50. Upon arriving at my destination, the driver billed me R85, to which I showed him the fee quoted on my app. He went on to explain about the duration of the trip that adds to the fee, which puzzled me because how can our fee prices differ. Mind you he was adament about his fee. Also note, the app shows a pre planned route. However he proceeded to travel his own route, calling it 'a short cut' but then afterwards realised that he needed to turn back and follow the prescribed route. If I'm to go by his theory of time affects prices, shouldn't he then cover the extra charges due to the 'short cut'?? I'm a regular Uber customer, but incidents as such puts one off due to the thought of drivers doing their own thing and overcharging customers. Like I asked the driver, what's the point of the app quoting the price before hand if it's going to change at the destination? Please shed some light.