In recent experiments by Bollweg et al. (Phys. Rev. Lett., (1996) 2774) a novel feature was observed in the far-infrared absorption spectra of quantum dots: the resonance frequency of the low-lying mode exhibits small oscillations in dependence on the magnetic field. By calculating the self-consistent far-infrared response of electrons in quantum dots, we can give an explanation for this behavior. For high magnetic fields, the occupation of electron states in the quantum dot can be related to a filling factor defined for the density in the center of the dot. We trace back the oscillations in the mode to an oscillatory filling factor dependence of the radial extension of the electron density.