The MAP adjuster works under all load conditions, not just at WOT, so even under part throttle the MAP sensor signal plays a major role in determining the A/F mixture.
When the ECU is in closed-loop mode (deceleration, light throttle cruise), it uses the O2 sensor as its primary signal for setting the A/F mixture instead of the MAP sensor so whatever signal it gets from the MAP sensor, the ECU will use the O2 sensor reading to correct the A/F mixture to stoichiometric.
Under open-loop operation (acceleration, warm-up), the ECU uses the MAP sensor signal as its primary input for setting the A/F mixture and the O2 sensor signal is ignored.