58 hours is 'min guarantee'. That is, fly 58, get paid 58. Fly 75, get paid 75. There is no 'overtime' rates per se between 58 and 75. The point that FYSTI is making is that when all three sets of crew fly 80 hours per month then their pay is going to be very close. In the J* and Virgin case, when the 'additional' rates are taken into account for day off flying etc, there is the potential for them to be earning more than the QF crew.
I'm not full bottle on the QF SH award so I may be out a bit too but that's the point I understand FYSTI to be making.