X forward and OpenSSH

Here is a simple question concerning port forward. I try to understand
why a "ssh -X" is working and why "ssh -R 6010:127.0.0.1:6000" is not
working:

$ ssh -v -x -R 6010:localhost:6000 benoit@localhost

benoit@localhost's password:
debug1: Authentication succeeded (password).
debug1: Connections to remote port 6010 forwarded to local address
localhost:6000
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: remote forward success for: listen 6010, connect localhost:6000
debug1: channel 0: request pty-req
debug1: channel 0: request shell
debug1: channel 0: open confirm rwindow 0 rmax 32768

$ export DISPLAY=localhost:10.0

$ xclock

debug1: client_input_channel_open: ctype forwarded-tcpip rchan 3 win
131072 max 32768
debug1: client_request_forwarded_tcpip: listen 0.0.0.0 port 6010,
originator 127.0.0.1 port 33251
debug1: channel 1: new [127.0.0.1]
debug1: confirm forwarded-tcpip
debug1: channel 1: not connected: Connection refused
debug1: channel 1: zombie
debug1: channel 1: garbage collecting
debug1: channel_free: channel 1: 127.0.0.1, nchannels 2
X connection to localhost:10.0 broken (explicit kill or server
shutdown).