  There is growing evidence that the estimations of the beaming Doppler factor
in TeV BL Lac object based on the Self Synchrotron Compton (SSC) models are in
strong disagreement with those deduced from the unification models between
blazars and radio galaxies. When corrected from extragalactic absorption by the
diffuse infrared background (DIrB), the SSC one-zone models require very high
Lorentz factor (around 50) to avoid strong gamma-gamma absorption. However, the
statistics on beamed vs. unbeamed objects, as well as the luminosity contrast,
favors much lower Lorentz factor of the order of 3. In this paper, we show that
for the special case of Markarian 501, the need for very high Lorentz factor is
unavoidable for all one-zone models where all photons are assumed to be
produced at the same location at the same time. Models assuming a double
structure with two different beaming patterns can partially solve the problem
of luminosity contrast, but we point out that they are inconsistent with the
statistics on the number of detected TeV sources. The only way to solve the
issue is to consider inhomogeneous models, where low energy and high energy
photons are not produced at the same place, allowing for much smaller Lorentz
factors. It implies that the jet is stratified, but also that the particle
energy distribution is close to a monoenergetic one, and that pair production
is likely to be significant. The implications on relativistic jet physics and
particle acceleration mechanism are discussed.
