It seems that the A pillars of D35 are consolidated than others.However none of their bonnets absorb the force well due to their body-on-frame structure.
I suspect two other reason than the A pillar : - the heavy duty frame is reinforced (330kg heavier vs standard ext frame) - the d35 ride seem to be a little bit upper
The D35 is a commercial vehicle were as the D15 is not, its like the Ford F series you have the lower end Ford F150 vs a Ford F550 they are designed to do different things....to my understanding of it