Luck or random chance don’t dictate the outcome of the viva. It’s not down to the examiners’ whims. It’s not an exam about “some” book and research. That book and research didn’t just appear. You didn’t just wake up to discover it was viva day. Candidates don’t just “suddenly” fail.
Eliminate the impossible and we’re left with the truth.
You created your thesis through talent, work and time. You earned your place in your viva.
You pass because of who you are and what you’ve done.