Re: Linux Filesystem Hierarchy Standard
Behind that is the possible case that /tmp is a ramdrive and small, while /var/tmp is expected to be on non-volatile storage and much larger. In the ramdrive case a reboot will inevitably wipe the directory even if the OS has no explicit step to do so.
Debian based systems like Ubuntu wipe /tmp on reboot only, where as RedHat based systems typically deleted from /tmp by cron job based on the last access time being a week or two ago.