Here's why:
* Atmospheric effects: The laser beam travels through the Earth's atmosphere, which can distort the path and affect the timing.
* Lunar surface reflectivity: The laser beam reflects off the lunar surface, but not uniformly. Different lunar features have varying reflectivity, adding uncertainty to the measurements.
* Limited resolution of the receiving equipment: The telescopes used to receive the reflected laser light have a limited resolution, which contributes to the overall error.
Current accuracy: The best Lunar Laser Ranging (LLR) measurements achieve an accuracy of about a few millimeters over the total distance to the moon.
Therefore, while LLR is incredibly precise, achieving centimeter-level accuracy is not currently possible.