Snell is just another standard that is a bit more strict than DOT. I noticed when i came from the streetbike world that people don't care as much about their helmets for offroad applications. I spent $600 on a damm good helmet for my motorcycle. It was snell and Dot approved and beyond that it was a proven helmet that protected much better than any of the others. Names like Arai and Shoei are top notch helmets for the street.
When i was looking for an offroad helmet i noticed the prices were cut in half or less. I mean good off road helmet is like $250-$300 and you can get a nice looking one that is DOT approved for around $100. So, i haven't come to the conclusion whether or not the standard for on road helmets is different than offroad (even though both of mine are DOT), or if they are just a cheaper made helmet.
here's a good description of the differences that i've copy pasted
"There are two key differences between a DOT and Snell approved helmet.
First, is the testing standards the helmets go through, and second, is the verification of the results.
The Snell Memorial Foundation (SMF) was founded in 1957 just after William Snell, A.K.A. Pete, died from massive head trauma from a race car accident. (SMF) is a non-profit organization that focuses on learning more about the components of head injuries, & how to prevent them by using helmets. Through their research (SMF) has developed the Snell standard for approving helmets. It is continually tested and updated every five years.
Although both testing processes are very similar, (SMF) requires their certified helmets to withstand a greater impact and absorb more force than the DOT approved helmets.
Both (SMF) and DOT standards drop the head forms on flat and round shaped anvils in order to simulate different types of impacts. However, Snell also uses an edged anvil that specifically tests the integrity of the helmet's shell.
DOT and Snell test each helmet two times at four different impact zones. DOT drops their helmets from 6 feet on the flat anvil and from a 4.5 feet on the round anvil. Snell drops the helmets from 10 feet and 7.5 feet on both anvils, which simulates a more severe impact.
While there is no guarantee that a DOT approved helmet has actually been tested, the purchase of a Snell approved helmet ensures that the product has been evaluated under scrutiny. A Snell-certified helmet also meets and exceeds the DOT standards."