This has been a bug in the past from back in arma1 if i remember rightly.
The throttle only engages in increasing rpms once 60% of traverse is reached.
That leaves only the top 40% of your throttle traverse as a usable area for increase and decrease of power.
This is why acceleration or deceleration is not constant and has a surging effect, meaning you cannot taxi and control your aircraft/helicopter speed with any type of accuracy.
Increase and decrease of power needs to be in increments and use the whole axis.