Sorry, Bob, you are confusing the "resize to width and height" setting, which preserves aspect ratio, with the "resize to fit dimensions" setting, which is supposed to set the exact dimensions, even if it changes the aspect ratio.
From the Lightroom help pages:
Dimensions - Applies the higher value to the longer edge of the photo and the lower value to the shorter edge regardless of the photo’s original aspect ratio. Specifying 400 x 600, for example, produces a 400 x 600 portrait photo or a 600 x 400 landscape photo.
I'm pretty sure this is a bug, but I can't figure out a workaround other than to write a Photoshop action to fix the dimensions after I export them.