Khomenko et al. estimate the mean magnetic field strength of the quiet Sun to be 20 G. The figure is smaller than several existing estimates, and it comes from the comparison between observed Zeeman polarization signals and synthetic signals from numerical simulations of magneto-convection. The numerical simulations require an artificially large magnetic diffusivity, which smears out magnetic structures smaller than the grid scale. Assuming a turbulent cascade for the unresolved artificially smeared magnetic fields, we find that their unsigned magnetic flux is at least as important as that explicitly shown in the simulation. The unresolved fields do not produce Zeeman polarization but contribute to the unsigned flux.Since they are not considered by Khomenko et al., their mean magnetic field strength has to be regarded as a lower limit. This kind of bias is not specific of a particular numerical simulation or a spectral line. It is to be expected when observed quiet Sun Zeeman signals are compared with synthetic signals from simulations. ; Comment: Accepted A&A. 4 pages, 0 figures