We carried out an experimental comparative study of the instrumental resolution function (IRF), using LaB6 powder, which is accepted as a standard reference material for line profile analysis and a high-quality SrTiO3 monocrystal, which was cut along the (001) crystallographic plane. The IRT extracted from LaB6 reflections systematically reached values about 30 divided by 40 % higher than the IRF from SrTiO3. The reasons for the differences discovered are discussed. The factors that contributed to the differences can be grouped as follows: dynamical effects of the diffraction on a monocrystal, out-of-focus effects when tilting a sample around the chi axis, and the surface roughness. Both sets of the FWHM values define the bounds of an area in which a correct IRF is expected.