  We interpret the large variety of redshift distributions of galaxies found by
far-infrared and (sub-)millimeter deep surveys depending on their depth and
wavelength using the B\'ethermin et al. (2012) phenomenological model of galaxy
evolution. This model reproduces without any new parameter tuning the observed
redshift distributions from 100 $\mu$m to 1.4 mm, and especially the increase
of the median redshift with survey wavelength. This median redshift varies also
significantly with the depth of the surveys, and deeper surveys do necessarily
not probe higher redshifts. Paradoxically, at fixed wavelength and flux limit,
the lensed sources are not always at higher redshift. We found that the higher
redshift of 1.4 mm-selected south pole telescope (SPT) sources compared to
other SMG surveys is not only caused by the lensing selection, but also by the
longer wavelength. This SPT sample is expected to be dominated by a population
of lensed main-sequence galaxies and a minor contribution ($\sim$10\%) of
unlensed extreme starbursts.
