I think it has to with weight and velocity. Seems like it would be the other way around though. Normally the faster the rifle the faster the twist. Like the difference between old muzzle loaders and the the newer inlines. And then high powered rifles have even faster twist rates yet. I would think the 12 guage would have the faster twist not slower. Maybe the 20 guage uses a lighter slug and is faster?

I would think if you were using on of the newer lighter saboted bullets you would want more twist.

Paul