Residual analysis to assess whether black hole mass or luminosity more fundamentally drives X-ray excess variance. Upper left: Residuals from the best fit of the log(σ²)–log(L_{2 − 10 keV}) relation plotted against log(M_BH). A weak trend can be observed (α = −0.10 ± 0.06), but the obtained p-value is 0.88, against the significance of the trend. Upper right: Residuals from the log(σ²)–log(M_BH) relation plotted against log(L_{2 − 10 keV}). There is no significant trend (α = −0.06 ± 0.08), and the p-value is 0.75. These findings suggest that for this dataset, neither M_BH nor L can be considered the ‘main driver’ of the variability against the other. Lower left: Same as in the upper-left panel but restricted to the RM-only sample. A 2.3σ significant negative dependence on M_BH is observed. Lower right: Same as in the upper-right panel but restricted to the RM-only sample. A 2σ positive dependence on luminosity is observed. This marginally suggests that M_BH is more predictive than the luminosity.