I write and hold spoon+fork righty. Everything else, I do lefty. I think many mothers do that. A kid writing lefty just doesnt look proper to them, maybe.
I am a lefty, and box orthodox when sparring with mates, though can box southpaw as well if I want to use a straight left.
Mike Tyson is a lefty and fought orthodox. Also, not a boxer, but Bruce Lee was right handed and fought south-paw - believed it was the better way to have your stronger hand forward. My kung fu shifu (pictured in my avatar) is a former Chinese and World sanshou champion and is also left-handed and fights orthodox. He did tell me that power can be generated in both hands though, through training. The jab is the most used punch so may as well keep the stronger hand forward. Good for hooks as well. Cotto and Tyson both have/had devastating hooks as their best punches. The reason is they are left-handed.