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There are two other aspects that you can take into consideration. Was the processing of the two images the same? I'm thinking more so with the cluster than the moon.
Another one is that of noise. Even if the images do look very similar, is there a difference in measured noise? Single exposures of the moon or of stars can be quite deceptive in evaluating brightness levels.
As has been mentioned already though, reflective surfaces do lose more light than refractive ones with modern day anti-reflective coatings.
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