Righto I know what you mean now and how its getting some guys panties in a twist.
The 79 has much more capacity than what is is plated for standard so the classification change is an arbitrary figure that they altered to suit the change in legilation.( remember also consumer registration costs are effected by stated capacty too)
Have a look at aftermarket GVM upgrades for other utes on the market compared to a 79. Some can only achieve a few hundred kg upgrade whereas a 79 can go an extra 750kg with just a suspension change, that demonstrates the reserve capacity built into them the others don’t have.
The 70 is a low volume production vehicle, the annual build numbers is about equivalent to a month of Hilux’s, it is teatering on end of production life as it was so they are not investing big coin (as
@shanegtr posted) into R@D like they do with other models. The last safety upgrade with the singlecab there was consideration of shelving the whole series rather than make changes.
Have a look at say a Fuso Canter, the exact same vehicle goes from 3.5t GVM to 8.5t without changing a single part except a differnt stroke of your pen, the Hilux is plated to tow 250kg less in Australia than what the same vehicle does in other countries, again just a stroke of a pen
The 79 series ethos makes a lot more sense if you look at it as a truck step down from the next category of Hino, Fuso Canter etc Whereas the other utes like say Hilux, BT50 etc get their DNA from the other end of the spectrum with passenger vehicles