mount hides space from du

peter:/mnt # mkdir dutest
peter:/mnt # cd dutest/
peter:/mnt/dutest # mkdir test1
peter:/mnt/dutest # mkdir test2
peter:/mnt/dutest # cp ~peter/testfile test1/
peter:/mnt/dutest # cd ..
peter:/mnt # du -shxc dutest/
201M    dutest/
201M    total
peter:/mnt # mount -o rbind dutest/test2 dutest/test1/
peter:/mnt # du -shxc dutest/
12K     dutest/
12K     total