Hello Thomas.
This is because at non-perfect accomodation, molecules gain speed easier than lose speed. This is what was explained in detail, and with concrete examples, in the discussion you linked.
(Please note for the following that you mixed up the annotation of the profiles, F141 is 0 acc. and F14 is 1)
With low accomodation (black profile) and perfect accomodation (red profile), the Maxwell-Boltzmann statistics is followed perfectly, as all particles thermalized perfectly at the last collision (for acc. 1) or desorption (acc. 0).
For in-between, outliers skew the statistics - the equation
results in particles gaining speed, due to the mathematical form of the equation.
The rest (higher temp than what the ideal gas equation predicts) is a result of this:
- The average speed of acc. 0.1 is the highest as they have the most fast particles distorting the statistics
- The average speed of acc. 0.5 is medium, as fast particles cool down quicker
- The average speed of acc. 1 and 0.1 is the lowest, as fast particles are thermalized instantly after a hit