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Ah, found a reference for the apparent speed change.

“As Henk Schuring and David Vogel have stated, the speed of light is slightly slower in the atmosphere, due to the index of refraction. The speed of light, at sea level, is roughly 299,703,000m/s. In a vacuum, the speed of light is set, by the new definition of a meter, at 299,792,458m/s.”

"... The standard deviation of the detection time distribution is pri- marily due to the contributions from the orientation of the retro-reflector array or target and the response function of the detector and timing electronics, while the shape of the laser pulse fired defines the theoretical minimum (with contributions from the detector and timing electronics). The contribution from the atmospheric turbulences become dominant at low elevation angles (around 10 ○ ) while LLR is typically performed at higher elevation angles (30 ○ to 40 ○ ) ( Currie and Prochazka, 2014). ... ... Hulley and Pavlis (2007) developed a new technique with the inclusion of horizontal gradients using a three- dimensional atmospheric ray tracing (3D ART) with meteorological data (NS and EW gradients) from the Atmospheric Infrared Sounder (AIRS), improving the SLR range residuals by up to 25% while ranging at low elevation angles (10 ○ ). For LLR ranging, the typical elevation angles lie between 30 ○ to 40 ○ where the effect is below 1 mm ( Currie and Prochazka, 2014). While this study (Hulley and Pavlis, 2007) can be a future improvement to the LLR data analysis community handling modern-day high accuracy data approaching the millimeter level, the ray-tracing technique would increase the computational time due to the large meteorological gridded datasets and become susceptible to temporal interpolation errors due to the availability of the data for each station. ..." https://www.researchgate.net/publication/277680224_Atmospheric_effects_and_ultimate_ranging_accuracy_for_lunar_laser_ranging