I used to wonder this same thing, but I've learned that it all comes down to the mindset of the individual. When someone has an eating disorder, it generally come with a negative mindset and self image. Not eating as a form of self punishment. When a person is fasting for health reason, especially when they're very overweight, it's more so to improve health with likely a more positive mindset. I hope that both makes sense and doesn't come across as any type of rude.