Screen backlight ON when connected to external monitor

[Sleepless]

I connected my X220 to an external monitor using the "Lenovo USB 2.0 Port Replicator with Digital Video". I'm choosing the "Projector Only" mode.
The laptop screen is darkened, which is what I want, but for some reason, the screen backlight is still on.
I did not have this issue with the X61s.
Any idea what's going on here?
Thanks!