Mechanical and electronic hand tachs still average out (smooth) the counts. But displays, depending on how they are programmed, are instantaneous so one per rev (or even partial rev) variations in speed can be displayed. If the bounce rate of your HD15 display is undesirable, reduce the update rate or increase the sampling time.
It is true that a higher accuracy encoder will show less bounce, but probably not to the point where the bounce is completely eliminated. This leads to the subjective question of, how much is too much? The lowest accuracy encoder could still give very good results if measured over more than one pulse.