Snell updated the testing some time ago, the AC above is correct and 2009 a piece was run in the NY Times, written by a Motorcyclist editor:
http://jalopnik.com/5582380/ho...
The fundamental flaw was that the Snell testing did not account for the weight of a S-M-L head and the fact that foam density needed to change with head size as head weight changed.
Revised in 2010, and later standards, they did correct the issue. To be fair, Snell was already going to update the standards, they do so every 5 years since 1970.
I don't own a Snell rated helmet because only 2 Modular helmets pass the Snell 2010 tests and neither is the right shape for my head.
I wear a Shoei Multitech, which is ECE and DOT approved.