chroot - df shows too much space on tmpfs

I have a server (SLES 11 running on a VMware hypervisor if that matters) with a tmpfs partition meant for mysql temporary tables, and I run mysql chrooted.

df -h gives me strange outpupt:

root@db12.lab:~# df -h /usr/chroot/tmp/
Filesystem      Size  Used Avail Use% Mounted on

tmpfs            77G   66G  7.9G  90% /usr/chroot/tmp

While mount goes like this:

root@db12.lab:~# mount | grep tmpfs
tmpfs on /usr/chroot/tmp type tmpfs (rw,size=512m)

The database runs allright and I don't see any fs-related errors in the log. I did try to stop the daemons and mount/unmount the FS, but it didn't help.

I wonder what it might mean and how such kind of problems is solved?

It doesn't affect anything, but it's somewhat mystical and I'd like it to go.