As I understand it the cruise control works in conjunction with the speedo. As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.
Its actually even less high tech, the ABS ecu produces a pulse count on the non drive by wire cars and speed value (its not in MPH or KMH) which goes to the cruise controller (non drive by wire) or the ECU (drive by wire) AND the instruments so the two are independent.
What happens is you activate cruise based on what the speedo is indicating and the ECU/cruise controller simply opens or closes the throttle to match the selected speed value (or adds or reduces fuel if it is a diesel)
So in summary, the cruise system is only inaccurate due to the reading of the speedo, add a more accurate speed indication and the cruise will naturally be more accurate.