Because you think it is.
One of the components of an anxiety disorder is rigid, black-or-white thinking, with lots of judging and discriminating and labelling. It’s not surprising that people struggling with OCD have developed all of these different classifications–that’s part of the illness.
The superficial elements of the specific fear are irrelevant. If you were worried about hitting somebody with a car and now you’re worried about accidentally poisoning your friends by under-cooking food you’re serving them, then you’re worrying about the exact same thing. There’s an uncertainty about causing people harm and you’re engaging in compulsive behaviors to cope with, check on, or control that uncertainty.
Tackle OCD at the most basic, fundamental level possible. Learning to accept each little fear separately leads to a very long, drawn-out, exhausting process. All anxiety disorders are essentially about engaging in behaviors to avoid something you don’t want to feel. A person controlling their eating and constantly checking the mirror or the scale in the hopes avoiding a particular feeling, is no different from the person constantly checking their email and writing and rewriting responses to their work colleagues in the hopes of avoiding a particular feeling.