It may be the color table and the order in which the shades of gray are in it... from white to black or from black to white.
I had similar problems with alpha textures at some point.
Try opening a new gray scale file, check if its color table is opposite to th eone of that pic and copy paste.. etc.. I dont remember the exact procedure anymore
Edit: open one of your old elev bmp:s which has worked ok and then paste this one on it and asave with new name... should work.... or at least you should find the possible color table problem.