Motor runs when disarmed

I am using the current version of Arduplane on a Pixhawk. I upgraded the "Hawk from Arducopter. I habitually keep a handwritten log of the installation and setup steps as I take them (checklist).

During my attempts to get the plane operational on the bench I noticed on several occasions after I disarmed that the disarm process caused the motor to run up to about 1/4 throttle. It could not be controlled by the stick or Mission Planner and the only way to stop it was to disconnect the battery.

I also noticed the same when switching from Manual flight mode to Stabilise, FBWA, FBWB.

The FPL showed the range for the RC inputs to be 1100 to 1900 which I assume are defaults. I can’t explain how those values got there post radio calibration as my Taranis has a range closer to 1000 to 2000.

When I recalibrated the radio with Mission Planner the running motor issue went away and everything then appeared normal.

While I am certain that I did calibrate the radio during setup it seems to me that there is now no evidence of that. Nevertheless there really ought not to be a situation where the motor can start up upon disarming for any reason.

I repeated what I thought was the lead up to this experience with a different Pixhawk just to be sure, with the same result.

The experience leads me to ask why the defaults are 1100 to 1900 when most radios produce PWM values of a greater range?