I can somewhat under stand the XM-8 and the SCAR doing a bit better due to there being quite a bit more open space between the bolt carrier group and the upper reciever allowing the dust/dirt to fall away and not get as bound up between the bolt carrier group and inside of the upper reciever, that only makes sense. Combine that with a good magazine design and you've got potential for very reliable operation in extreme environments.
I honestly think that since the M4 type rifles have such tight tolerances in the relationship between the bolt carrier group and the upper receiver, that is where the "problems" lie in regards to reliability in extreme environments, that along with bad magazines make for a multitude of problems. I've said a thousand times,"bad magazines are the root of all evil." I can't help but think the M4 was sadddled w/ poor mags during this test. I feel that if the SCAR,M4,HK416 all had the same mags (all have the USGI or HK-HR) things would have been a bit more representative.
I also don't see how big an improvement the 416 is over the M4, when they both have essentially the same close tolerances between the bolt carriergroup and upper receiver, sure the 416 has the gas piston, which is most probably a great improvement when comparing the shorter 10.5" bbl versions of the AR w/ 416, but in the longer barrel versions not as much of an improvement. I know the introduction of the hot gases into the upper reciever can cause the lube to dry up, but this would not be such an issue if it weren't for the tight tolerances.
Extreme enviroments require a looser relationship between the major moving parts and the interior of the upper reciever, pure and simple. Look at the FN FAL, when the Brits first adopted it in the late 50's/ early 60's, they found that it had too close interior tolerances between the bolt carrier and upper receiver, so they had great big zig-zag channels milled into the bolt carriers of their SLRs, once this was done there was somewhere for the sand/dirt to go rather than binding up the bolt carrier in the upper.
I think that if a minimal amount of R&D was carried out to "recontour" the exterior of the AR's bolt carrier group to allow more space between the upper receiver and it, without sacrificing the needed bolt carrier weight and still allowing sufficient contact w/ the upper receiver to maintain good lock-up and to retain good accuracy, a good rifle would be made even better.