I don't believe 78% of the passengers from London are finishing their journey at Perth
They aren’t necessarily. Many may be choosing a few days stopover before continuing East (or home in the other direction), perhaps others continue elsewhere by road or on Skippers/Rex...
Again, access to Staff travel and/or EFF will show you that the load factors in/out of Perth are very good.
I have heard that even management were surprised at Perth/London/Perth itself providing much of that load, but there it is.