Hi,
Every channel input is scaled to a -1 / 0 / +1 range so whatever the input type and min/max/neutral values, after the scaling all inputs are know as -1/0/+1 in code.
For safety reasons the -1 (min throttle) is needed for arming, the Throttle neutral value is set around 4% of the whole stick range.
If you set the throttle neutral like others you get only a half stick range used for throttle.