I've owned 3 Android phones and 3 iPhones and 1 iPad and, just from my personal experience, the Android devices were buggy as hell and crashed constantly. The iOS devices were generally stable and reliable.
The problem with tracking stability is that one has to do it over the lifetime of the device, and I suspect this study doesn't do that. In all three cases with the Android devices I've owned, they started off with about 5-6 months of stability comparable to iOS. But that gradually deteriorated and the devices started acquiring a lot of weird behavior--sudden restarts, crashes, freeze-ups, etc. This behavior would increase in frequency over time to the point where the devices would become nearly unusable. In the case of iOS, no such change in stability would occur.
I can't explain that other than perhaps assuming Android is prone to corruption to the OS. Whatever the cause, I've never experienced this on iOS.