Curious what you did with the saddle and bar position? Interesting comment about knees. Legs are a big component of drag. This tends to work against short cranks since they will tend to keep the quads more vertical. But if you can keep your knees in better with shorter cranks, I can see that helping.

I raised my saddle to match full leg extension (so going from 170 to 162.5 I raised saddle 0.75cm) and left bars alone.

Yeah it's kind of equivalent to just raising the seat, from an aero point of view -- maybe worse because you maintain more leg extension at the top -- which is usually bad, but clearly that was more than offset by something else, and I'm guessing that was some combination of my knees being better controlled at the top of the pedal stroke, and otherwise being more comfortable in aero position so holding a better position.

Could also be that the increased drop lowered drag more than enough to compensate.

Yeah I should have mentioned that from other testing I know that's not what it was.