  We have found that the error-signal frequency corresponding to the coherent
population trapping resonance can be displaced from that of "$0-0$" transition
unperturbed by the optical field, although the frequency is not sensitive to
changes in its intensity. We consider the double $\Lambda$-system of levels
interacting with the asymmetric polychromatic optical field to demonstrate that
this effect arises due to intensity nonlinearity of the error-signal frequency
shift. The experiment with $^{87}$Rb atoms in Ar-N$_2$ buffer gas atomic cell
shows how the displacement value depends on different parameters. The possible
influence of the effect on the clocks' frequency stability and reproducibility
are discussed.
