It is absolutely normal to expect some oscillation at encoder pins, particularly if it is featured with high resolution. Keep in mind that although its output is at digital format, its movement is linear, and considering that sensing mechanism is based on the obfuscation of a light beam through a fringe printed at a crystal wheel, it is not guaranteed that it will stop precisely at the center of the slice. There can happen vibrations at imperceptible levels for us, but enough to sparkle it status.
What you can do in order to remedy that, is treat this event by software, such as taking the average, disregard very small variations below determined speed, etc...