From owner's estimations, ~90% of the battery's stated capacity is actually available for use. The 10% is not so much "fail-over" capacity as it is a buffer to keep the battery away from the extremes of charge/discharge states, where most of the degradation occurs.
Your cell phone battery has no problems charging to 100%, since you want to get as much energy (and therefore use time) in there as possible for the weight. However, charging to 100% harms the chemistry, and after a few years the battery no longer lasts as long. That's fine for a cell phone - part of it is planned obsolescence, part of it is the reality that a lot of people won't keep their phone more than a few years.
In an EV you have the luxury of maintaining a charge buffer, since the added weight and cost is fairly minimal and people have a much higher expectation of long-term performance.
So usage pattern + design (thermal management) + energy management which does prevent the user from destroying it = significantly different performance degradation profile.