I think unattended-upgrades only runs once a day by default, and the repository index could have changed since then. Different results on different machines can be because unattended-upgrades didn't run at the same time, or from the same mirror (and mirrors take a while to propagate changes). Generally speaking you should always run an apt update directly before installing new packages.

Some upgrades require human input like when core service config files upgrades are offered. (ex. would like to update /etc/samba/smb.conf with the maintainer's version or keep your own?)

In my experience this can occasionally cause background apt processes to hang while they wait for your answer to that kind of question. There is a debconf trick you can try. debian_frontend=noninteractive. You can create your own cronjob, as root, that runs a script with this export command, apt update, then apt dist-upgrade -y.