Comparison of the 2-keV luminosities for objects with multiple X-ray observations. The best fit is consistent with the bisector line (solid line). The dispersion parameter due to variability is δ_var = 0.08 dex. When distinguishing between the high- and low-luminosity subsamples, we see that δ_var is higher for lower luminosities. The difference between high and low luminosity is not visually discernible, as the total dispersion for the two subsample is nearly the same. However, while in the high-luminosity sample the observational uncertainties on the fluxes can almost completely explain the overall scatter, in the low-luminosity subsample a significant contribution associated to variability is required to explain the scatter we observe. Luminosities are derived from photometric fluxes, assuming a standard flat ΛCDM model. We note that, as we are comparing luminosities for the same object, the results do not depend on the chosen cosmological model.