Dithering
Dithering is the attempt by a computer program to approximate a colour from a mixture of other colours when the required color is not available. Typically, dithering occurs when a colour is specified for a Web page that a browser on an operating system can't support. The browser will then attempt to replace the requested colour with an approximation composed of two or more other colours it can produce. The result may or may not be acceptable to the graphic designer. It may also appear somewhat grainy since it is composed of different pixel intensities rather than a single intensity over the coloured space.

Only 8 used colours but the image is dithered, image size 1,246 Kb

Raster image editing packages such as Adobe Photoshop or Macromedia Fireworks offer the option of saving the GIF image with a small colour palette and use dithering to smooth the result. As explained GIF compression works at its best when working with full tints over areas. Dithering "sprinkles" many pixels in the boundary areas to suggest gradual overflow. GIF compression will need more space to encode this. It is better avoid dithering. It doesn't help in reducing the file size and certainly doesn't improve the quality of the graphics.