TaylorSPL

We do a ton of these so we have a loose window on a stand to shrink/cut on.

From http://www.xpel.com/faqs/

Quote

This is a very common occurrence with Paint Protection Film of all brands.  It has to do with the difference in refractive index between the layer of film and the paint on the car.  It is most noticeable on compound-curved surfaces because the film must be stretched over these areas to allow it to be installed without wrinkles.  When the film is stretched, it becomes slightly thinner in those areas which causes the phenomenon seen through polarized glasses.  

It can also occur on flat surfaces when the installer needs to use a drying solution (as opposed to a slip solution) such as alcohol and water to get the needed bond to the paint.  This is due to the difference in viscosity of the fluids and their effect on the adhesive’s properties.

Polarized glasses have this effect on a number of different multi-layer surfaces (not just paint protection film) made of glass or plastic.