If McLaren aren't taking advantage of the highest allowable nose height then they must know something we don't or have something up their sleeve. OTOH perhaps their nose might rise mid season.
So th enose can only be so high and then at some mark the fuselage can(or must?) be another, higher hight? How do you determine where that mark is?
I hope that soon they will word the regs in such a way as to make it impossible to have a nose that looks like that. Of course, they never outlawed Alain Prost's honker.
I'm at work so I can't see the vid that acosmichippo posted, and I hope someone else has not said all this already, but I will try to explain as best I can in plain text. I hope I understood your question correctly. I read that the noses have to be lower because the FIA is worried that if car A hits car B in a T bone type of crash, the nose of car A could hit the driver of car B straight in the head. So with the lowered nose, that's not an issue any more. Think about how vettel's nose speared into the side of button's car at spa 2010. The nose could have possibly hit button in the head if vettel's car were a little farther forward than it was.
The regs say that the nose height can be up to 625mm (I think) above the reference plane to a certain point at the front of the car, but then the nose has to drop down to a height of 425mm by another point a little farther towards the front of the car, which is why you're seeing the stepped noses. So the teams with stepped noses built the nose to be as high as possible until the point where the regs say it has to drop down to 425mm. Mclaren said

that, we're gonna just make a nice smooth slope down, instead of doing the stepped nose idea.
again, I hope i understood your question correctly, and anybody feel free to correct me if i'm wrong