I don’t know, but my guess is that they allow time for having to take extra pictures when the radiographer is having a bad day. I was once told off very testily for swallowing during an MRI, and they had to re-do some images, which the radiographer grumpily conceded were just about good enough despite what he clearly saw as my wilful non-cooperation.
It sounds as though yours (in contrast) all went smoothly.
Yes, some people can stay very still, some less so - it can make a big difference in the time it takes! I’ve always managed to keep very still, though last time was difficult because they were in such a rush that they didn’t wait for me to get into a comfortable position, and started the scan and I had to stay in that position the full time.