I think the explanation is simple physics. The Volt's grill is way down low where the big rocks live. And because the grill is lower, rocks on a flatter trajectory may have a cleaner shot at getting through the grill than ones on an upward or downward arcing path. And theoretically, because these rocks aren't burning off speed or kinetic energy by fighting gravity as hard, they may be able to carry more energy into your radiator. Combine that with the almost 1" slot between the grill slats, and you have problems.
I'll admit to this being all based on a 30 year + memory of high school physics and no testing, so if I'm wrong, then I'm wrong. But at least it sounds good in theory. Doesn't really matter any ways, the solution is to keep the rocks out of there in the first place.
Funny thing- Take a look at the grills and openings in front of radiators in cars you see in parking lots. You'll be astounded at how wide open so many are. Some just have wide open holes. So why aren't there radiators getting punctured all over the place? Maybe they are, but I'm not hearing about it from the public at large. And when I stopped by a couple of radiator shops to check out the Volt problem as it became apparent a couple of years ago, the guys there didn't make anything of the huge holes. No problem, no big deal, not extra work from them. And none of them looked like they were working 80 hours a week or living in a palace.
Steve