The method of charge for the Decathlons at ACFT is based on Tacho time. Therefore at the end of your flight if the Tacho time is say 0.8 hours then you pay for 0.8 hours of aircraft hire but you log 1.0 hours (add 0.2 hours) in your flight log book. The additional 0.2 hours that you add to your log book time is to cover average taxiing/waiting time etc when you are still in command of the aircraft so you are still entitled to log the time. So you are getting more log book time than you are actually paying for!
If you would like any further explanation give the guys at ACFT a call on (08) 9417 1927 or visit them and they will be only too happy to explain it to you.