  Radio and X-ray fluxes of accreting black holes in their hard state are known
to correlate over several orders of magnitude. This correlation however shows a
large scatter: black hole candidates with very similar X-ray luminosity,
spectral energy distribution and variability, show rather different radio
luminosities. This challenges theoretical models that aim at describing both
the radio and the X-ray fluxes in terms of radiative emission from a
relativistic jet. More generally, it opens important questions on how similar
accretion flows can produce substantially different outflows. Here we present a
possible explanation for this phenomenon, based on the strong dependency of the
jet spectral energy distribution on the magnetic field strength, and on the
idea that the strength of the jet magnetic field varies from source to source.
Because of the effect of radiative losses, sources with stronger jet magnetic
field values would have lower radio emission. We discuss the implications of
this scenario, the main one being that the radio flux does not necessarily
provide a direct measure of the jet power. We further discuss how a variable
jet magnetic field, reaching a critical value, can qualitatively explain the
observed spectral transition out of the hard state.
