It will add or remove fuel to keep your mixture at 14.7.The only way to change your mixture in closed loop is to fool the ECU by changing the O2 sensor output voltage.
After 10psi Les must be breaking into open loop mode and seeing the "tune" mixture.

Re-read your post Grip. I didn't think it would be possible for the ECU to request a different mixture at different engine loads. If it could then we could throw away our widebands and tuning cables and run a feedback loop all the way to redline.