DAF95XF:
This is where we need Rog
I’ll finish that sentence for you Stu… 
… to over-complicate something already adequately explained by Malc’s reference to David Lowe. 
The only thing I’d add to what’s already written is that the extra allowance on each axle comes in very handy when a load is of irregular shape or is composed of materials of different densities, provided that the permitted GVW isn’t exceeded. 