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The IACV can only let in more air above and beyond what you allow in with your base screw setting. In a normal setup, the screw is adjusted to let in enough air to hit a ~650rpm. The ECU idle speed setting for an MT is 700rpm. When you turn the IACV back on, the ECU sees that the idle is less than the 700rpm target and increased the duty cycle of the valve to open it, letting in more air, and increasing the idle to 700rpm. If your base setting is higher than the ECU setpoint, the ECU will see that the idle is too high and keep the IACV duty cycle at 0%, which essentially renders it's non-functional. In your case, the only time the IACV is actually probably doing anything is:
1 - During cold starts when the ECU targets an idle of up to 1250rpm, depending on how cold the coolant is. In this case, the ECU would increase IACV duty cycle until the higher cold-start RPM is achieved
2 - At high coolant temps, the target idle speed is 1000rpm I think, presumably to increase fan speed and airflow. If you were to experience a high enough coolant temperature, the ECU would again increase IACV Duty Cycle until your ~850rpm base idle is increase to ~1000rpm.
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