Why are some of the Landsat TM browse images so blocky?

Apparently, the color stretch used by the processing system to create browse images for Landsat TM images does not perform well in areas of low contrast. The resulting browse images are of very low quality and probably do not indicate the real quality of the full resolution image. Imagery containing snow, ice, and clouds may experience this problem. There is nothing the USGS Global Visualization tool can do to correct this problem since the original browse images show the problem.