The way it is explained on the CCF website is as follows, while far from an empirical, scientific test, I do think it does a pretty good job of conveying the idea.
The explanation for the accuracy enhancement has to be the flash suppressor eliminating "dirty air" from being created ahead of the bullet by expansion gas. If you watch a slow motion video of a bullet leaving a barrel you will notice for the first couple of inches out a barrel, the expansion gases actually speed forward past the bullet. We surmised that either one or both of the following two scenarios were taking place - 1st, expansion gases were creating dirty air ahead of the bullet or 2nd, as the expansion gases break past the rear edge of the bullet's base, they torque the bullet and give it a yaw and destabilizing it. When you have the vortex flash suppressor on the barrel, while the bullet is in the flash suppressor, the expansion gases are channeled off where they can't torque the bullet and by the time the bullet has exited the flash suppressor, whatever expansion gas remains behind the bullet has lost most of its energy (gas having very little mass dissipates its energy quickly).
The phenomenon is just too widespread to be discounted, from pistols to match rifles, it seems that almost any gun you put a vortex on will get better accuracy afterwards.