Because the color space is a different shape. It's hard to predict what happens to the clusters (their shapes) after you do that kind of non-linear transform. You might want to download and install ImageJhttp://rsbweb.nih.gov/ij/download.html and the Color Inspector 3D pluginhttp://rsbweb.nih.gov/ij/plugins/color-inspector.html so that you can visualize the 3D color gamuts. You will be glad you did because it is very enlightening and informative. MATLAB does not (yet) have this capability.
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