Interesting. I've never exported a previously imported terrain and compared heights so I cant comment specifically.
However I have successfully exported terragens own internally generated procedural terrain as a Lightwave mesh, used that mesh as a basis for motion paths and models, and then re imported those back into Terragen. Everything worked and was in the right place. So I'm think there was no miss-match in the height data.
Two things come to mind, firstly the further away from Terragens own internal zero point (0,0,0) the more inaccurate it becomes. There are work arounds for this but I'd have thought that Planetside would have taken this into account when they made the importer. ALso the reverse may be true, the Terrain may only work properly when positioned correctly on the Planet.
Secondly I don't use the Heightfield Export LWO operator, I place a camera (set to ortho, size to the dimensions of my terrain) vertically above the center of my terrain and use the "Lwo micro exporter" located at the bottom of the render nodes far right tab. It gives greater flexibility of resolution, takes longer and created HUGE files
(Oh and if you switch of the atmosphere and place the sun behind the camera gives you a great texture map )Sorry I couldn't be of more help, good luck
Richard