It was my understanding that manufacturers self-certify to ANSI and CPSC standards, possibly on "pre-production" prototypes. Snell on the other hand buys the helmets at retail and oversee the testing themselves.

So the standards may be similar but the oversight and adherence to the standards is likely to be higher from a Snell-approved helmet.

But as has been mentioned, many manufacturers don't sell Snell-approved road helmets.

The above is my recollection from research done a couple years ago.

-Tom in SoCal