  We show that, observationally, the projected local density distribution in
high-z clusters is shifted towards higher values compared to clusters at lower
redshift. To search for the origin of this evolution, we analyze a sample of
haloes selected from the Millennium Simulation and populated using
semi-analytic models, investigating the relation between observed projected
density and physical 3D density, using densities computed from the 10 and 3
closest neighbours. Both observationally and in the simulations, we study the
relation between number of cluster members and cluster mass, and number of
members per unit of cluster mass. We find that the observed evolution of
projected densities reflects a shift to higher values of the physical 3D
density distribution. In turn, this must be related with the globally higher
number of galaxies per unit of cluster volume N/V in the past. We show that the
evolution of N/V is due to a combination of two effects: a) distant clusters
were denser in dark matter (DM) simply because the DM density within R_{200}
(~the cluster virial radius) is defined to be a fixed multiple of the critical
density of the Universe, and b) the number of galaxies per unit of cluster DM
mass is remarkably constant both with redshift and cluster mass if counting
galaxies brighter than a passively evolving magnitude limit. Our results
highlight that distant clusters were much denser environments than today's
clusters, both in galaxy number and mass, and that the density conditions felt
by galaxies in virialized systems do not depend on the system mass.
