The landscape image was indeed spot metered.
This could account for the dark
exposure. If the central focus point was chosen as the spot, and the
meter catered mainly for the sky section of the horizon, then this would account for the dark
exposure.
Pixel luminance values of the slight cloud mass along the horizon are roughly in the 140 ballpark, so this makes the most sense.
And if mpb's comment that the images have been cropped in any way is true, then this also makes it harder to assess the exposures.
Also I noticed that the OP uses Ps Elements(ACR7.1) .. in my experience with ACR via Lightroom 4, is that it usually tend to render a
raw file more bright when compared to the rendering on the camera and on the
Nikon software I use.
If you have access to
Nikon's ViewNX2, I'm wondering how different the image rendering looks between the two softwares(assuming that you shot in NEF format!!).