Because they may not even know they are speeding since the tolerance for speedo's, if it hasn't changed very recently is +-10%, and they do drive under WELL under these days. Last time I visited the old man in Sydney I spent the whole time doing 60 in 80 zones, and people breaking approaching GREEN lights nearly hoping they went red, because of these new(not so new any more) safety cameras. Seriously, the joints becoming a joke, couldn't be more glad i moved out of the joint.
You are right about people pushing the limits on roads with slow limits, I set my speed off my GPS +5% on some roads, particularity out this way where just over the boarder its safe to do 130 and on the same road here with the same traffic its dangerous, go figure.
On another note, the NT brought in speed limits on the Stuart (130) and noticed an increase in the accident rate, ended up trialling unrestricted again. Fatigue being more dangerous.
Rant over!