In that more people just suddenly drop dead without warning.
Anorexia is not healthy either, especially long term. I think that most anorexics are identified by others before they have serious damage. I have read though that anorexia has the highest mortality rate of any mental illness though.
Anorexia varies on how severe it is. If you fast completely, you will drop dead within a couple of months. Other anorexics do eat what might look like a "healthy diet" to an outsider, but it is not for them because they continue to lose weight even when they are underweight. Obviously the healthy diet anorexics would do better longer than the complete fasters, especially if they are also taking vitamins.
You do come to a point though where you just don't have reserves of stuff anymore. Your body suffers as you are chronically malnourished vitamin wise. You get cold. You get tired. You get moody. You don't have spare fat, protein, or carbohydrates floating around in your body like the average american. You come to the point where your body can't handle trying to be normal if you continue with what you are doing. If you restrict even more, you face serious health problems. Your organs are protein too, after all. I think that's perhaps why long term anorexics have a higher death rate. It becomes progressively more damaging with each passing month.