Oh, I was going to have to sleep on it.
In bed, I knew the twill proportion was the key to solving this mystery; it was something about the proportion of the warp thickness vs the weft and how the shape was going to come out, but it was a conundrum.
When I got up I just knew the three warp ends (natural and two pinks) were supposed to be threaded together! So, even though it has 30EPI, the draft pattern is coming out fatter and squarer as if it's 10EPI.
But the threading is so much quicker now, so I forgave me.