Gender is a social construct, and is currently a pretty big part of how our society and language is structured. And being a social construct it doesn't perfectly describe how everyone experiences the world. Most people fit pretty solidly under man or woman, cis and trans. Some people don't, and it can be hard for someone to describe where they fit using a language that's so strongly built around binary genders. I don't know exactly why he chose to describe himself that way, but if I had to guess I'd say he doesn't feel entirely masculine or feminine, but leans more towards masculine. That's probably the easiest way to describe it. Technically the most accurate word for my sexual orientation would be gynesexual, but nobody knows what that means so typically I just say I'm bisexual because it's easier and is close enough for most situations. It's probably just easier, especially being a public figure, to say "trans man" and make it simple.