I've always thought no one is born ambi just like no one is born a genious. Some people are more prone to it but very rarely is there a natural in these fields.
For me it all started when I was a kid and I broke my hand and couldn't brush my teeth. I didn't care about writing so I pretty much got away from doing any real school work for a while but I love brushing my teeth. So every since I can remember I brush my teeth with my left hand, which also led me to shaving with my left hand. I still throw a ball and write with my right but I cannot brush my teeth or shave with my right.
So to me it's kind of bizarre. For most things I usually start with my left hand, when I spar in tkd I use a south paw stance cause it just feels more comfortable.
Chucking I make a concious effort to learn EVERYTHING AND PRACTICE EVERYTHING with my left first. Cause I know that my right hand will always be able to do it better with half as much practice. Then my left hand gets jelaous and quits.
As much as I like to think I'm ambi when it comes to chucking I'm not. I'm an extreme right DH and unless it's sparring, shaving, or brushing my teeth I have to go to just as much extreme to make sure my left keeps up with my right so my doubles don't look lopsided.
As for when other people say they are ambi I have great respect for that cause as far as I'm concerned it doesn't come natural in wny way shape or form and you have to work extremely hard till you reach the point where you start automatically picking up your chucks with you LDH(less dominant hand).