Two Internal Compasses instead of one?


We are trying to calibrate our Rover compasses and we came across the following paradox:

We have connected an external compass to our Ardurover which acts as our primary compass. We do know that Pixhawk 4 has an internal compass which we will use to just in case of failure of the main compass.

The issue is that by observing the status tab of the Mission Planner we see values to mx,mx2 and mx3. What is happening and we see values on mx3 field? As far as I can see Pixhawk has only one internal compass, am I wrong?

We moved forward and also check the full parameters list and we see COMPASS_DEV_ID,

Could anyone explain me this behavior?
Thanks in advance!