The materials used to make photographic polarizing filters are never perfect, but the nearer they are to perfection the higher the price. Käsemann polarizers are supposed to be made from better materials, and the laminate is supposed to have better edge sealing than other polarizing filters.
These are the likely sources of differences in quality for a circular polarizer:
The polarizing foil will have varying degrees of attenuation of the crossed light – ie the polarization direction it is supposed to be stopping. The maximum attenuation varies between materials, as does the spectral evenness – the degree of attenuation at different wavelengths. Hence different colours of polarized light may be attenuated to different degrees, resulting in a colour cast. Typically the blue and red light is less attenuated than the green. The maximum attenuation is up to about four stops for the better quality polarizing foils. (This means that crossed polarizers can have up to about eight stops of attenuation, by the way.)
The polarizing foil will attenuate, to some degree, the light that it is supposed to be passing. The degree of attenuation will vary with wavelength. Achieving high, spectrally-even attenuation of unwanted light and low, spectrally-even attenuation of wanted light is difficult. A perfect polarizer would lose one stop of light of all wavelengths when the incident light is unpolarized or circularly polarized.
The ‘delay plate’ or ‘quarter wave retarder’ that makes up the second stage of a circular polarizer will also be less than perfect – mainly because it is very difficult to make a suitable delay plate that works at different wavelengths, so only one wavelength will be circularly polarized, all others will have a mix of circular and linear (ie elliptical) polarization. This shouldn’t matter too much.
Best,
Helen