Generally 1/9 will stabilize up to 68 or 69 grain, at least at .223 16" barrel velocity.
Given that the .22 cal centerfire bullet markets biggest users are in AR platforms, my wild ass guess is that that these bullets were built to stabilize in a 1/9 twist 16" AR chambered in .223/556.
It's also possible that since 1/7 and 1/8 have become the standard AR platform barrel twists (in 22 caliber), these bullets were just built for those.
