No spin. You can do simple arithmetic. Having glass on two sides rather than one presents twice as much chance of something breaking:
3GS = 2.1% cracked the front glass
4 = 3.9% cracked either the front glass or the rear glass
So to me, that sounds about right. Twice as much opportunity for something to break, and twice as many breaks.
I fail to see the big deal in these stats.
That's of course a different question from the technical issue of whether having glass on both sides was a good idea.
I think that is the point. Putting glass on both sides doubles the chances of cracked glass. While it might look nice, IMHO, it was a poor design.
Of course, Apple fans seem to prioritize “coolness” over functionality and practicality, so for Apple's target market, the dual glass design might be a plus.
I don't mean that as a knock on Apple fans, just an observation. A more durable, but less esthetically pleasing device is not necessarily superior. It is a matter of preference.