  Approximating a real manifold by an idealized one requires to calibrate the
parameters characterizing the idealized manifold in function of the real one.
This calibration is a purely conventional process and can generally be done in
several ways, leading to different fittings. In practice, however, all possible
fittings cannot be considered as representative of the real manifold.
Approximating the real metric of the universe by the FLRW metric would be
adequate only if both corresponding structures, defined by the space-time
interval, are equivalent on large scales. This requirement puts some
constraints on what would be a representative FLRW metric. We show that the way
how measurements on SNIa are interpreted to determine the evolution of the
scale factor implicitly define the calibration process, and that this one is
compatible with the aforementioned constraints. On a theoretical point of view,
this indicates that the as fitted FLRW metric would indeed be representative of
the real one. On a practical point of view, however, we show that a bias in the
measurements could invalidate this conclusion. The bias comes from the fact
that SNIa are not randomly distributed over space, but are probably mostly
located in regions were matter is largely present, i.e., in overdense regions.
We explain how this bias could account for the apparent accelerated expansion
of the universe, without needing to introduce the dark energy assumption. We
show in particular that this bias leads to an inadequate fitting of the FLRW
metric, resulting in the appearance of a new term in the evolution equation of
the related scale factor, being equivalent to the cosmological constant.
