That's a really nice link. So the amalvalve isn't really an open/close valve, it can also be somewhere in between. That explains it a bit more.

The amal map for the L6 shows that the boost is slowly reduced over the whole rev range. It also says the L8 runs higher boost as the L6 and also more boost at higher rpm. This explains why the 4x4 makes max power at 6250 rpm and the 2wd at 6000 rpm. It's probably also the reason why the 4x4 got up to a higher top speed, although the Ford figures show it shouldn't have.