Why do I have an eating disorder when I don't have all of the symptoms?

For example, I very rarely count calories and I never weigh myself. Or is it like physical health conditions where you don't need to have all of the symptoms to have the condition?

It sounds like maybe you're worried about your weight. Have you been diagnosed by a psychiatrist with an eating disorder? A GP could diagnose a physical symptom of an eating disorder such as obesity or malnutrition, but not the eating disorder itself. If you haven't had a diagnosis, but you feel you may have an eating disorder, you should speak to your GP about it and they can refer you if necessary to a specialist. If you have had a diagnosis, the basic answer is that disorders involve a range of symptoms not all of which will be manifest in every case. So yes you could have the condition without having all of the classic symptoms.