Several factors come into play. First is lock up. Boltguns generally lock up much tighter than other actions. Second is lock time. The less time between the sear breaking and primer ignition, the better. An example of a slow lock time can be found in a Winchester Model 94 lever action or a Ruger #1, both of which use hammers to strike a firing pin. Third would be the ability to glass bed the action and free float the barrel. Having nothing touching the barrel generally (not always) results in better accuracy. Glass bedding simply ensures no action movement within the stock.

The statement that bolt actions are the most accurate action is not an absolute. I have a Ruger #1 that is simply amazingly accurate. Also, consider that most people really dont care to shoot any better than 1-1.5 MOA. Hey, if you can hit a 4 inch circle, you can kill a deer. Some of us, however, are somewhat anal about accuracy (myself included). If one was to line up 10 of every kind of action available, bolt actions would probably win the accuracy test.