[gentoo-user] why does revdep-rebuild object to mounting /var on /mnt/var ?

I use lvm and use it for /var.
In fstab I have
  /dev/vg/var  /mnt/var  ext4  defaults  0 2
I also have
  lrwxrwxrwx 1 root root 7 Aug 31 16:13 /var -> mnt/var

(Similar setup for /tmp and /opt)

This has worked ok but revdep-rebuild is not happy

root@E6510 cache # revdep-rebuild
 * Configuring search environment for revdep-rebuild
 * Working directory expected to be /var/cache/revdep-rebuild, but it is /mnt/var/cache/revdep-rebuild
root@E6510 cache #

I don't intend to fight revdep-rebuild so will change and mount directly
onto /var, but I wonder what is the concern.

Should I also mount directly onto /tmp and /opt?

thanks,
allan