Mustang MkIII?

[Voyager]

So, some background to start, this all started with me trying to figure out how to deal with FW-190D-9's in a P-51. I was messing around in Il-2 compare v4.07, when I noticed the MkIII had high performance at sea level. I used HardBall's Aircraft Viewer 4.08 and found the following top speeds at sea level listed:

P-51B: 577km/h
P-51C: 580km/h
P-51D: 592km/h
Mk.III: 636km/h

What is it that makes the Mk.III 40km/h faster at sea level than any other Mustang in game? In fact, looking at it, it looks as though it has the highest speed at sea level of any of the conventional engined planes. Did the British use a lower altitude supercharger on their version?

Harry Voyager