While it is true that Io's rotation period is very close to its orbital period, it's not perfectly synchronous. Io has a slightly faster rotation period than its orbital period. This results in a phenomenon called orbital resonance, where the gravitational pull of Jupiter and the other Galilean moons (Europa, Ganymede, and Callisto) slightly influences Io's rotation.

This resonance is a major factor contributing to Io's incredible volcanic activity, as it creates a significant amount of tidal heating within the moon.