Why are some non OEM windscreens DOT stamped and the windscreen reveals waviness after parked in sun. Also, during rainy days and when A/C is on, the glass gets fogged up but the fogging does not come out after wiping. Seems like it is between the glass layers as wiper blades dont remove it.
I thought DOT standard was a minimum standard so all these issues won't exist.

You are wrong when you say SOME non OEM w/s are DOT stamped. ALL are if they are legal.

To be legally installed in the U.S., a w/s must be DOT certified. DOT certification has nothing whatsoever to do with w/s "quality" issues like proper bend, optics, how soon if ever it will delaminate, etc. It only relates to issues of withstanding breakage.