  The most accurate method for modelling planetary migration and hence the
formation of resonant systems is using hydrodynamical simulations. Usually, the
force (torque) acting on a planet is calculated using the forces from the gas
disc and the star, while the gas accelerations are computed using the pressure
gradient, the star, and the planet's gravity, ignoring its own gravity. For the
non-migrating the neglect of the disc gravity results in a consistent torque
calculation while for the migrating case it is inconsistent. We aim to study
how much this inconsistent torque calculation can affect the final
configuration of a two-planet system. Our focus will be on low-mass planets
because most of the multi-planetary systems, discovered by the Kepler survey,
have masses around 10 Earth masses. Performing hydrodynamical simulations of
planet-disc interaction, we measure the torques on non-migrating and migrating
planets for various disc masses as well as density and temperature slopes with
and without considering the disc self-gravity. Using this data, we find a
relation that quantifies the inconsistency, use it in an N-body code, and
perform an extended parameter study modelling the migration of a planetary
system with different planet mass ratios and disc surface densities, in order
to investigate the impact of the torque inconsistency on the architecture of
the planetary system. Not considering disc self-gravity produces an
artificially larger torque on the migrating planet that can result in tighter
planetary systems. The deviation of this torque from the correct value is
larger in discs with steeper surface density profiles. In hydrodynamical
modelling of multi-planetary systems, it is crucial to account for the torque
correction, otherwise the results favour more packed systems.
