becuasr there is a load o them. if there is any bind at all then the servo is going to have a hard time centering. The servo is acuated by an electric motor oviously and any electric motor only generates power/torque when it turns, so it can have a hard time finding a single spot. this is usually noticable on new models when everything is really tight. You do need to eleviate any binds in any way shape or form. and more ARF's even though they'er better that they were a few years ago more offten than not, are set up wrong. I'll usually get a minor buzz on one or more servo for the first 2 flights then it's gone since things losen up. I've found the biggest culprit are new CA hindges being really stiff.

Also, what kinda of servos are you using and what kinda of power sourse are you using. Some of the digital servos don't like a 6V battery.