Here's a totally off the wall thought from my feeble mind.
What if a higher twist rate results in the bullet having more contact with the material it impacts?
Here's the logic behind it. If you have a zero twist rate the path of the bullet is a straight line. If the bullet spirals it's a longer path through whatever it comes in contact with. The higher value of kinetic energy is what carries penetration forward but the spin effect increases the area of contact by spiraling.
Here's a way to imagine it. If you poke your finger against a desk the area of contact is limited. If your finger twists slightly before stopping the amount of contact is increased.
Increased contact = more expansion
I'm not an engineer so I could be totally wrong and I'm OK with that.
