Brightness/Letterboxing on Zoomplayer

[nx6]

This should be an easy one for home theater buffs.

I'm watching a 16:9 show on ZoomPlayer, and I'm getting slight letterboxing on my widescreen display (as I should, it's a 16:10 LCD panel after all).

My problem is the letterboxing isn't the darkest thing on the screen.

I can see areas of the show are actually shades of black darker than the letterbox frame ZoomPlayer adds in fullscreen mode. I know this isn't a contrast trick of the eyes, because when these dark areas are up against the edges of the picture I can see the letterbox frame stand out slightly from it. I've tried setting the aspect ratio for the video to 4:3 so I could rule out blacklight leak on the display, and the bars are still visible even though they are now a couple inches into the screen on the sides instead of top and bottom.

Is this a sign my brightness is too high, or is it normal? I can lower the brightness on my video card settings slightly to get rid of it, but since I was able to see image area that was darker than the bar before, haven't I now lowered my shadow detail as well?