Left panels: Γ, ṁ and D_F histograms of the full AGN and XRB samples (blue and red-dashed lines) with additional subsamples: “AGNc” (light blue) is obtained reshaping the original ṁ distribution to be as narrow as the one in XRBs (using its 16th and 84th percentiles), albeit keeping the same median as in the full “AGN” sample; the same reasoning is applied with the Γ distributions to select “XRBc” (orange) from “XRB”; instead, “AGNc2” (dark azure) is obtained by selecting sources with ṁ compatible within 0.4 dex with the 5th–95th interquantile of “XRB”, whereas “XRBc2” (dark orange) follows the same reasoning, selecting Γ compatible within errors with the “AGN” Γ distribution. The vertical dashed lines in the middle-left panel represent the location of the 1st and 99th percentile of multiple XRB ṁ distributions, obtained by converting the fit T_in spanning a_* = 0 − 0.98 and m = 5 − 10, while we highlight with a red-dashed histogram the one obtained with a_* = 0.5 and m = 5.8 (see text). Right panels: mass-normalised log L_disc − log L_cor plane for the full AGN and XRB samples (top) and the above-mentioned “c” (middle) and “c2” (bottom) subsamples. In the latter case, the uncertainty in determining ṁ for XRBs (vertical dashed lines in the middle-left panel) reverberated in the dark-azure points spreading in the log L_disc − log L_cor plane, covering the same dynamic range of the full AGN sample. The computed observed scatter of each relation is shown on the side, with the same colour coding as the data: 0.30 ± 0.01 dex and 0.43 ± 0.02 dex for the full samples of AGNs and XRBs, respectively; 0.30 ± 0.01 and 0.33 ± 0.02 in the respective “c” subsamples, 0.31 ± 0.01 and 0.36 ± 0.02 for the “c2”.