Solid black lines: cumulative distributions of residuals in the linear fits to the log L_opt, pk vs. log L_X, pk scatter plot. Each line refers to one of the six linear fits shown in Fig. 3. Dashed orange line: predicted distributions assuming the optical peak luminosity is perfectly correlated to the X-ray peak luminosity and that the observed optical flux, from an optically thick slab-like region, is attenuated by projection effects (see text). Dashed green line: same as above with an additional attenuation due to limb darkening. The fact that the observed scatter is smaller than predicted by these assumption, even ignoring measurement uncertainties and flare-to-flare variations, indicates that the assumptions on the optical emitting regions are not correct.