The airspeed indicator never measures speed directly. It measures a pressure — and that pressure depends on air density just as much as it depends on how fast the aircraft is actually moving.
Climb an airplane to cruise altitude and something strange happens to its airspeed indicator: the number on the dial stops matching how fast the airplane is actually moving through the surrounding air. It isn't a malfunction. The indicator is doing exactly what it was built to do — it's just that what it measures, and what most people assume it measures, are two different things. Understanding the gap between them is one of the first things every pilot and flight-performance engineer has to internalize, because the two numbers can differ by a large margin at altitude, and confusing one for the other produces real navigation and fuel-planning errors.