The Servo library uses timer interrupts to control its outputs, not hardware PWM, if other interrupts are
happening (like the normal timer0 interrupts that drive millis()), there will be jitter due to interrupts
interfering.
Other libraries exist which get round this problem, but none of the alternatives can drive 12 servos
at once(*) which is Servo library's main strength.
(*) 48 servos on the Mega