Because uname(1) shows the kernel version which is shared with the host. TrueNAS core 13.3-U1 is the latest version but you will have to download the manual update package and update manually. I would highly recommend you do that since you are running 13.4 jails on a 13.0 base system and there are some package incompatibilities that were introduced when FreeBSD 13.3 was released.
So I guess that was a long way of saying, no don’t ignore it and update your base system.