The seattle robotics site below has a lot of stuff on servos. I think there is a article there on how to modify a continous rotation servo such that the controllable rotation speed range is much broader. It think it has indepth info on the resistor/capacitor relationship in the servo that determines the range over which the servo rotation speed can be controlled.
Actually, it isn’t really different at all - you’re just seeing a behavior that is usually a lot harder to see in normal servo operation.
As the servo’s output shaft (and thus the feedback pot) approaches the commanded position, it decelerates the motor, so as to have less overshoot when it gets there. The farther it is from the set point, the faster it runs the motor, and the closer it is, the slower it runs. This normally occurs in a fraction of a second, so it’s hard to observe in an unmodified servo.