I think the standard was set up to allow use of convertible Cat I / Cat II implements. This is not a very common type of implement, but when the QH standard was developed, perhaps they thought that would be the normal approach in the future. Standards are meant to be as flexible as possible. If they had made it purely Cat I it would have limited use with non-specific implements.
Edit: I don't think the HF and RK hitches are EXACTLY the same but either will do the job at a better price than the big names. Just be prepared to do a little grinding on the hooks if you get one that's a little off.