It says to choose twist rate based on the heaviest bullet you intend to shoot, which means that according to them a 9 twist is good for bullets up to a maximum of 75 grains rather than the minimum.
That's the way I read it which is the way it works in my rifles, too. I get very good accuracy with 69 Sierra's but my groups triple in size with 70 grain Barnes. I know the Barnes is long for weight, but have never tried a 75 and believe it would show an accuracy deterioration at 223 Remington speeds.
