We show that for some ultrathin multilayer structures the proportionality of the longitudinal Kerr ellipticity signal to the magnetic film thickness does not take place. Instead, thinner magnetic layers give even twice larger contributions than the thicker ones located a few monolayers deeper. This effect is demonstrated both experimentally and theoretically on ultrathin Fe/Cr/Fe sandwich systems prepared on GaAs(001) surfaces. Such a behavior is explained by a depth variation of phase relations between the optically reflected wave and the wave generated by the magneto-optical interaction. This mechanism becomes very effective close to the phase where the magneto-optical ellipticity cancels.