Why is S car fuel mileage so poor?

[Enzeder]

Why are the S cars mileage numbers so low?  I figured consumption would be 
a matter of engine load and efficiency.  As both engines are pulling the 
same weight (and shape in the case of the S8), the load should be roughly 
similar.  So does that make the S cars 15% engines less efficient?

Take the S8 for example.  Two naturally aspirated V8's of the same 
displacement and hauling the same weight.  Looking at highway numbers (as 
it's fairly steady state):
S8 vs A8 (both 2001)
21 vs 24 mpg
4024 vs 4068 lbs
360 vs 310 HP
317 vs 302 ft/lbs
11.0 vs 10.8 comp ratio
13.9 vs 14.2 ¼ mile
6.3 vs 6.7 0-60 mph
An aside: 345x30mm vs 312x25mm (323x30mm for the A8L)


As for the S6:
S6 vs A6 4.2 (both 2002)
21 vs 25 mpg
4024 vs 4024 lbs (surprised by this)
340 vs 300 HP
310 vs 295 ft/lbs
11.0 vs 11.0 comp ratio
14.5 vs 15.0 ¼ mile (14.7 for the 2.7tt w/6-speed)
6.5 vs 6.7 0-60 mph (6.0 for the 2.7tt w/6-speed)
Curiously, both have 321x30mm front rotors.  I expected the S6's to be bigger.

Numbers taken from the AudiWorld site.