[BaNaNaBoy69]

Apparently, the heat from sun exposure through the windshield causes the oils from the dashboard vinyl/plastic trim to leach out and coat the inside of the windshield mixed with dirt and dust. That's why people use the windshield screen when parked to prevent this and keep their vehicle cool in the summer. Also, the trick to cleaning the windshield is after you've cleaned it, wipe/buff it down again with a different new, clean microfibre cloth.