A GIF image with less than 256 values may have some extra symbols when symbolized with a Unique Values renderer. For example, if the GIF only has four values (0, 1, 2, 3), eight values will appear in the legend. The extras (4, 5, 6, 7) will be represented by a black symbol.
This occurs when no statistics are present for the data. ArcMap uses the largest pixel value to determine the bit depth of the image. If the image does not use all values possible in the bit depth, ArcMap fills in these remaining values with black patches. ArcMap uses bit depths of 2, 4, 8, 16, 32, 64, and 128.