Would someone please clear the whole "CrankHP/WHP/AWHP" thing up for me?

[S4DIT]

Ok, so I've been following all these threads for two months now and I can't seem to get my mind around these figures:

Crank HP
WHP
All wheel HP

I know that there is some drive train loss when transferring the "crank" HP to the wheels, but why does everyone seem to post different figures?? Isn't the "WHP" the only thing that matters? Why don't all the "crank" people just subtract the 15% loss (or whatever it is) and give us the WHP up front?

But here is my real question: Obviously we all are driving the B8 S4 which is all wheel drive - how does all wheel drive play into this? If someone says they got 400 AWHP does that mean I should multiply 400 by 1.25 to get the "total" or "real" HP?

And, what is the difference between "Peak gains" and "max gains"??

My apologies in advance if these questions have been answered in previous posts - I'm sure they have...