We examine the special case of spectroscopic ellipsometry measurements in the vicinity of the fundamental absorption edge of wurtzite ZnO and GaN when the optic axis is perpendicular to the sample surface. It is demonstrated, both by calculations and direct experimental data, that the isotropic dielectric function (DF) derived by means of the isotropic model does not lie between the ordinary and extraordinary DFs and is not their mixture. In general, the isotropic DF also cannot be considered as being approximately identical to the ordinary DF. We observe that their difference may become significant and, in addition, shows a complicated spectral behavior, especially in a range of excitonic absorption. The discrepancy between the isotropic and ordinary DFs of ZnO exceeds 15% at room temperature. For GaN, this effect is considerably smaller. However, the discrepancy might not be neglected if the temperature is sufficiently low.