Hi Tracy,
Unfortunately very little is logical about UK rail fares since privatisation 30 years ago. The specific issue here is between who sets the fares. On the Cricklewood line it is GTR. They charge by zones within London, and there are no off-peak fares to encourage use of PAYG. London Overground is part of TfL and their stick to promote use of PAYG is to charge very high cash prices.
This is all well and good until you need to use a paper ticket for a days out offer.