I had a rock (or something) smack my windshield today (woke me up!). No damage. I got lucky.
My father was heading back from TN a couple of weeks ago. He told me he had a chip in his winshield on his new Rav 4 on the way back. He called an hour later to say the chip isn't an issue anymore... a larger rock had made a 1' crack across the windshield.
In FL, if you have comprehensive coverage, there's no deductible to have the windshield repaired or replaced. So, soon as my father got home, he made the call, the repair guy came to his house, and fixed it on site. Looks like new.
If you just have a few nicks in yours, they can do a pretty good job fixing them these days. I don't think it has anything to do with the strength of the glass. Some people just get lucky.