Three different histograms of the instantaneous Stehl ratio, measured from three adaptive optics telescopes, were recently published (J Opt. Soc. Am, November 2010) by Gladysz et al., in a paper on the use of the probability density function (PdF) of adaptive optics for exoplanets detection. Their PdFs present values of the skewness coefficients of different signs. Surprisingly they are the same kind of histograms that we found many years ago, measuring the time evolution of the intensity at the focus of a large lens collecting an atmospherically degraded laser beam, many years ago. We describe the explanation we gave of our results (Ciolli, Consortini et al., Applied Optics, May 1977) and suggest here a similar one.