Could you highlight where the pixels are lost or gained? Comparing all of your shots against the default 16:9 shots, everything lines up on the very top and bottom.
I tweaked the screen change section to hopefully make more sense.
This is what is happening, I think.
1920x1200 has a viewable picture range of 1920x1080. However the 1920x1200's 1920x1080 picture range has an additional row of vertical pixels added to the top and bottom of the picture range as compared to a native 1920x1080 resolution. The sides are the same meaning there is some stretching.
When I mean stretching I am talking about a few pixels out of 2+ million but it is still there. Unless something else is going on.