I have to agree with Funky on this one. I too believe a large batch of sub-par glass made it on to some phones. I'm sure that there are quality controls in place to ensure the glass meets certain standards before Corning will release it, but there is no way they can test each and every individual glass unit before it ships. QC is merely a numbers game, therefore less than perfect products can (and do) get through the checks.
Check out this example:
My wife, two brothers, two sisters-in-law, and myself all have the SGS3. And all the phones vary by carrier and/or purchase dates. With the exception of my wife, all of us are rockin' the SGS3 case-less. One of my brothers had his phone in his lap while getting out of his car and the phone fell from a height probably no more than 2 feet and the screen shattered (spider web effect on 1/4 to 1/2 of the screen.) Then, one of my sisters-in-law was getting into her car one day and had her hands full so she put some stuff on top of her car including her phone. Needless to say, she forgot her phone on top of her car and drove all the way home from work. When she got home and realized what she had done, she called her phone and much to her surprise a lady answered it, and explained how she found it while walking her dogs only 2 blocks away from my sister-in-law's house. My sister-in-law met the lady to pick up her phone and found the glass to be in perfect condition and only a scratch and small dent on the side where the phone likely made impact when it fell.