Page 81, paragraph 3 from the auger bit catalog verified that the double twist is for soft wood, the single twist is for hard wood, exactly the same as the Kreg Pocket hole screws.
The problem here is they never define what a single twist is or what a double twist is...... On page 82 we see a bit that is good for both woods and then on page 87 the bit is good for both woods.....
Then they talk about "coarse" and "fine" screws being applied to either single or double twist.....
Now for Kreg the fine thread vs the coarse is easy to see but we would not say that a fine thread is a double twist...... anyway I'm confused.
I have both types pictured and I can not say I ever thought about which to use for want wood. I do know that my uncle had a hand drill press (you cranked it hand) and he had the bits with more twists per inch..... and the power ones you buy have the more twists per inch so maybe it has something to do with speed of rotation too??????