Probably polarization, but I'm not sure - where is this question coming from?
Coming from M.Lepselter at : firstname.lastname@example.org. Is that your question?
I thought you might have a plasmon answer but polarization is too broad an answer. The box is reflecting from all angles and a 2.45GHz photon is still 12 cm.
A microwave oven is a Faraday cage. The leak from a door slit will exceed the typical leak rate from the window when the size of the slit exceeds the distance between rulings (mesh size) on the window. You can't simply compare the wavelength to the opening and use that as a hard limit to determine whether or not there will be leakage. There is always some leakage and it is a smooth function of the wavelength and separation between conducting paths.