And it gets a bit confusing when they say this is:
"Each flight is paid for after the flight is completed and prices vary depending upon the amount of time actually flown, i.e. brakes off time to brakes on time".
What they mean is that your lesson is not always going to be an hour, and therefore not the exact same price every time. As someone said, if you sit holding for 15 minutes, and you still want an hour in the air, your going to be paying for 1 hr 15 mins, or thereabouts so the cost is variable. Plus if you land away, you can never predict how long you may spend taxying or holding.