Greetings,
Mr. m. Mr. dh is correct regarding the "spotting" being a result of the cooling process of the glazing material, in your case, glass. Part of the tempering method involves blowing air on the hot glass as it exits the lehr (
https://en.wikipedia.org/wiki/Lehr_%28glassmaking%29) . Those "spots" are areas where the air hit the surface of the glass. Most, if not all, automotive style tempered glass is manufactured via this method. The reason you haven't seen this in your car may be the angle of the glass or the orientation of the glass in the frame. If you tilt your head those spots will most probably become more or less visible depending on the angle. NOT suggesting you drive with your head crooked but it will demonstrate that viewing angle
does have an effect.
Perhaps a different brand of polarized glasses may help or the next time you get a prescription pair of spec's, ask the optician to orient the polarization in a different direction (I have no idea if this is possible but it may alleviate some of the eye strain).