There are regulations for vivas, and expectations for vivas, but all vivas are different. Some are more different than others.
My viva took four hours. That makes it different from most, by comparison a long viva.
I gave a presentation to start my viva. That made it different, though I didn’t know it at the time.
I was stood for all of my viva, which is really different! I started my presentation, and my examiners neatly segued into questions. I just stayed where I was by the chalkboard. I didn’t feel uncomfortable or wrong (although close I started to feel pretty tired towards the end). This was just how my viva was.
All vivas are different is another way of recognising that every viva is unique. Every viva is a custom, one-off examination process, that follows expectations and guidelines and academic culture, but is still unique.
Unique or different, this doesn’t mean bad. Just… different!
Some vivas are more different than others, but the purpose is always the same.
Make sure you know what the purpose of the viva is: what your examiners are there to do and what you are there to do. Know this before you get there and you’ll feel far more comfortable even if your viva is atypical.
