Debouncing
When striking a keyboard key, the key oscillates against its contacts several times before settling. When released, it bounces again until it reverts to its rest state. Although it happens on such a small scale as to be invisible to the naked eye, it's sufficient for the computer to register multiple key strokes inadvertently.
To resolve this problem, the processor in a keyboard "debounces" the keystrokes, by aggregating them across time to produce one "confirmed" keystroke that (usually) corresponds to what is typically a solid contact. Early membrane keyboards limited typing speed because they had to do significant debouncing. This was a noticeable problem on the ZX81.
Read more about this topic: Keyboard Technology