What does ‹above› mean in df output? (How to get out of chroot?)

Question

FreeBSD

Noob question.
In FreeBSD 9.0 i386, this is the picture (after a failed NanoBSD image creation - but that's beside the point):

Filesystem                                         Size    Used   Avail Capacity  Mounted on
/dev/ada0p2                                        7.5G    2.2G    4.7G    32%    /
devfs                                              1.0k    1.0k      0B   100%    /dev
<above>:/usr/src                                    15G    9.7G    4.7G    67%    /usr/obj/nanobsd.custom/_.w/usr/src
<above>:/usr/ports                                  15G    9.7G    4.7G    67%    /usr/obj/nanobsd.custom/_.w/usr/ports
<above>:/usr/obj/nanobsd.custom/ports/packages      22G     17G    4.7G    78%    /usr/obj/nanobsd.custom/_.w/usr/ports/packages
<above>:/usr/obj/nanobsd.custom/ports/distfiles     22G     17G    4.7G    78%    /usr/obj/nanobsd.custom/_.w/usr/ports/distfiles
devfs                                              1.0k    1.0k      0B   100%    /usr/obj/nanobsd.custom/_.w/dev

I understand this is the result of chroot?

Logoff doesn't change anything. After a reboot, things get even worse - the original directories /usr/src /usr/ports... are overwritten and everything has to be started over.

The question is, what does this mean and how to get out of this lossless mode?