The reason it works this way is that each jailshell process on a system that supports the "Mount limited /proc" option for the "Jailed /proc mount method" tweak setting sets up this limited proc environment by using the CLONE_NEWPID flag when launching a shell or other process (CLONE_NEWPID documentation can be found here:
clone(2) - Linux manual page) and then remounting /proc based on that new PID namespace. This means that each jailshell process will be in a PID namespace distinct from all others, even if they're running as the same user.