Hmm...I wondered this, as well.
So if a .223 1 in 9 twist barrel is the general rule for 55 gr. bullets, then 100 gr. .308 bullets need just slightly slower twist?....1 in 10? and slightly less than double the weight of .223 at 55gr.?.....coincidence?
Im sure this is not a linear rule!
Sorry, engineer coming out in me....