  Different determinations currently suggest scattered values for the power
spectrum normalization on the scale of galaxy clusters, sigma_8. Here we
concentrate on the constraints coming from the X-ray temperature and luminosity
functions (XTF and XLF), and investigate several possible sources of
discrepancies in the results. We conclude that the main source of error in both
methods is the scaling relation involved, in particular the way its intrinsic
scatter and systematic normalization are treated. For temperature derived
constraints, we use a sample adapted from HIFLUGCS, and test for several
sources of systematic error. We parameterize the mass-temperature relation with
an overall factor T_ast, with 1.5 \le T_ast \le 1.9. After marginalising over
the range of T_ast, we obtain a 68 per cent confidence range of
sigma_8=0.77^{+0.05}_{-0.04} for a standard LambdaCDM model. For luminosity
derived constraints we use the XLF from the REFLEX survey and explore how
sensitive the final results are on the details of the mass-luminosity, M-L,
conversion. Assuming a uniform systematic uncertainty of +/-20 per cent in the
amplitude of the mass-luminosity relation by Reiprich and Bohringer, we derive
sigma_8=0.79^{+0.06}_{-0.07} for the same standard LambdaCDM model. Although
the XTF and XLF derived constraints agree very well with each other, we
emphasize that such results can change by about 10--15 per cent, depending on
how uncertainties in the L-T-M conversions are interpreted and included in the
analysis. In order to achieve precision cosmology, with this probe, it is
important to separate the uncertainty in the scaling relation into its
intrinsic and overall normalization parts.
