As such, when the wheel is rotated, the mouse things it has stopped scrolling, but when you release the wheel, it settles in the bump, causing a slight rotation forward or backward. This can be exacerbated with age due to wear. The problem is that the bumps inside the wheel (“resting positions”) are not always perfectly aligned with the slits.
Moreover, you’ll notice a series of thin slits on the face of the wheel through which a small beam of infrared light passes, and the mouse detects the interruptions as the wheel rotating. If you open a wheel-mouse and look at how the scroll-wheel mechanism works, you’ll see that it has a series of bumps inside the wheel and a spring that presses against the bumps to cause the click-y feeling.
Assuming that the amount of back-scroll is only a single click of the scroll-wheel and not half-way up or down the page, then what you describe is typical of wheel-mice, especially older ones.
