Without reading the book or it's sources.
There is a vast number of support personnel in Afghanistan that are not exposed to combat. "Fighting in Afghanistan" gives most people the impression of soldiers on patrol, which is far more dangerous than being a non-combat support personnel.
A large majority of the American motorcycle population use motorcycles only to engage in more risky activities (bar hopping or thrill seeking). This skews this death-ratio upwards from what this more risk-adverse (mature adventure riding) population experiences.
I have no doubt that the statistics are mathematically accurate, but they do not give an accurate depiction of reality.
My motorcycles are my only form of transportation. I am a mature, conservative adult rider with no accidents or speeding tickets on a bike, but that doesn't mean I'll live forever. If I am tired, if I am late for work, if it is raining, if I am angry, if it's 105 degrees outside, if I had a (single) beer after work, if it's 30 degrees outside, I will be getting on the bike and riding because that is what I love to do. The preservation of life is not the primary goal of living. I accept that death will catch up to me no matter what I do. How do you want to die?