Much more simple than you would think
Canberra is a point that is overflown by aircraft flying between Sydney, Melbourne, Brisbane and Adelaide. Whereas not many aircraft will overfly Perth at altitude while going to other locations.
To relieve my confusion, are you trying to say that there are less "Contrails" over Perth, due to there being a lessor number of aircraft over-flying that point?
If so, I would agree with your answer. Otherwise, I'm really confused!
