The magnetic properties of ultrathin ferromagnetic films are studied by means of Spin Polarized Low Energy Electron Microscopy. Measurements of the onset of ferromagnetic order, distribution and shape of magnetic domains, magnetization direction and their change are now well established standards for this technique. Here, the asymmetry parameter has been determined as a function of film coverage and energy of the incident electron beam. It reveals oscillatory behavior which is usually described as due to the quantum size effect (QSE). We explore the origin of the characteristic features observed in the asymmetry curves and distinguish between the QSE oscillations and other phenomena influencing the shape of the asymmetry curves. As an example we discuss the asymmetry of ultrathin iron films grown on the W(1 1 0) surface.