My apologies, you are correct about the seating capacity of the Railbed cars.
However,
The economy (coach) cars on this train only hold 51 seats, as compared to a V-2 that holds 60 seats. I suspect that the cars are smaller than Amtrak cars, as the Queensland service is 3'6" narrow gauge. Thus, a V-2 sleeper shell would fit more than 19 Railbed seats. The problem with 2-bed private rooms is that beds are taken out of sale if you don't have enough couples to fill the rooms. Open-plan lie-flat seating allows the operator to keep every bed available for sale right up until departure.