It' mostely signedness but also from the specification: OpenGL's pixel map, color table, convolution, color matrix, histogram, and min/max are NOT performed on the HILO components or texture offset group pixel groups. (I'm not sure wether these operations are performed when shaders are enabled, though.) Also, even GL_HILO8_NV is internally represented by 16 bits per channel (at least according to the spec).
Wined3d doesn't use them because direct3d does not support them. So no need to worry about them :-)