Winning the lottery is lucky: you buy a ticket, or lots of them, and maybe yours is the one that wins. There’s no skill, it happens or not.
Winning a race is fortunate: you develop skill, and even if there are other skilled people taking part your skill wins out. It isn’t luck, because you didn’t leave it to chance.
One of these descriptions is like the viva, and one is not, despite both being about situations involving a great many people.
If you’re lucky, you did something but it wasn’t in your control really. It could have been anyone else who succeeded, and what you did didn’t particularly matter. If you’re fortunate, then something good has happened, but what you did made a difference. Success in the viva is fortunate, I think, because it comes down to your developing talent through the PhD and what you are able to show on the day.
This is how I put the line between lucky and fortunate; you might define them differently, but I think you take my point. If you’ve read a lot of posts on this blog then you’ll know it’s a recurring theme for me: success in your PhD and viva is down to your talent and is not just good luck. This is important for the story you tell yourself afterwards. Not “I was lucky with the questions I got,” but “I was fortunate that I had done the work and could answer their questions well.”
You’re fortunate, you’re not lucky.