  We constructed the cumulative luminosity distributions of most supergiant
fast X-ray transients (SFXTs) and the classical supergiant X-ray binary (SgXB)
IGR J18027-2016 by taking advantage of the long term monitoring of these
sources carried out with Swift/XRT (0.3-10 keV). Classical SgXBs are
characterized by cumulative distributions with a single knee around
$\sim$10$^{36}$-10$^{37}$ erg/s, while all SFXTs are found to be significantly
sub-luminous and the main knee in their distributions is shifted at lower
luminosities ($<$10$^{35}$ erg/s). As the latter are below the sensitivity
limit of large field of view instruments operating in the hard X-ray domain
($>$15 keV), we show that a soft X-ray monitoring is required to reconstruct
the entire profile of the SFXT cumulative luminosity distributions. The
difference between the cumulative luminosity distributions of classical SgXBs
and SFXTs is interpreted in terms of different wind accretion modes.
