Anybody can Register or Certify a person in anything they want. It's just a term that is not restricted in usage and it's up to the NREMT to choose which term they want to use. My patch is sitting in the envelope it came in and I haven't looked at it in 4 years so I couldn't tell you what mine says.
As a logical extension of your question: a license is something only a government licensing agency can grant you. So a State Licensed EMT is someone who is granted powers by the state to perform certain skills. This is just like the State Licensing you to fish (to allow you to catch fish, or try to catch fish depending on how good you are!) or the state giving you a drivers' license (allowing you to operate a car).
You can certify EMTs if you want, but unless it's respected it doesn't mean a thing, and it certainly doesn't mean a thing in the eyes of the law.