It only maintains ratio when dragging scaling, and only in the event that you started with a multiple of 16x10.
When you have a non 16/10 resolution and select letterbox, mouse resizing uses black bars to maintain the current aspect ratio.
Furthermore there are no 16/10 resolution in the supported resolution arrow list (640x400 isn't in the cycle), arrowing cycles through all aspect ratios. Black bars are used in those resolutions but the pixels are still F'd, I guess that's a bug though.
Following resolutions are offered:
640x480
800x480
800x600
1024x600
1024x768
1280x720
1280x768
1600x900
Following resolutions are 16/10:
640x400 (vanilla res x1)
720x450
800x500
960x600
1024x640
1120x700
1152x720
1280x800 (vanilla res x2 - My Favorite)
1440x900
1600x1000
1760x1100
1920x1200