Ever heard of Yield management systems, McIce, or know how they work?
If BA charged that on every seat on every flight, there would be no BA. I suspect what you saw then was the price for the last five seats on the aeroplane and the QF ones were for the last 15 or so.
You book early enough on QF you can beat
VB fares, notice how
VB have gone away from the lowest fare type advertising of late?