The controller sticks have an axis, exactly the same as a yoke would have.
The idle point on a controller (when you aren’t moving it) is the centre of that axis. I would wager a guess that it’s simply the app registering that as the centre, or in other words, 50%
This can happen a lot with controllers where using incorrect software can result in this axis of having a value of 0 to 1 rather than -1 to 0 to 1. Infinite flight doesn’t officially support controllers either from what I’m aware.
If you are using a controller, I’d personally recommend trying to use the triggers to control throttle, as you can map them individually.