Even though a helmet manufacturer might have Snell approved helmets because of the random testing that the Snell Foundation does that manufacturer can lose their certification if the quality of their helmet goes down.
The idea of the random testing is to guarantee the consumer that the product that receives the certification is the same product that they are buying and that the quality of the helmet has not gone down.
The Snell Foundation randomly tests helmets that have their seal of approval to make sure that manufacturer continues to meet their rigorous standards.
If a helmet fails the test the Snell Foundation will request three more helmets to see how they hold up. If they fail the test, the manufacturer can lose their certification and the Snell Foundation can ask for their decals back.
The Snell Foundation does try to work with the manufacturer before taking away their certification so that the helmet manufacturer can fix the problem and keep their certification.
However, before the Snell Foundation will allow a manufacturer to put their decal on a helmet it must pass all five of the Snell Foundations tests with flying colors.
Because the Snell Foundation is a non-profit organization who have a set of voluntary standards set by the Foundation not by a government agency their standards are higher than even the DOT standards for helmets.