I don't know Josh, they're just real comfy - except on trails, which you can read more about along with some other exciting stuff in my latest blog posts (7 & 8) from a truly disasterous weekend.......
I'm a 14 stone forefoot / midfoot striker, I have over 150 miles on my Kinvara's and they look virtually new. I'd be surprised if I don't get 400 miles out of them at least. I reckon a pair that's screwed after 120 miles must be a duff pair.
I've used mine for 2 half marathons, several long runs (16 - 18 miles) inc some on trails, loads of shorter tempo stuff and I'll be running my next marathon in them. They do everything superbly, they are incredibly comfy, and are the fastest shoes I've ever had (they leave my Newton's and DS Trainer's for dust).
I'm running half marathons at around 90mins and next marathon goal is 3hrs 10mins. My training is anywhere between 6:30 pace for tempo stuff up to 8:00 pace for long slow runs.