That is because the remaining 40 colors vary on Macs and PCs. By eliminating the 40 variable colors, this palette is optimized for cross-platform use. It is used by Netscape Navigator, NCSA Mosaic, and Microsoft Internet Explorer. For a millions of color monitor, it takes 24 bits of data to describe the color of each pixel on the screen, where 8 bits go to each red, green and blue. This can give each color a brightness level between 0 and 255.
Grays have equal amounts of all three colors.
Check the image on the left to see that the HTML number for these three colors are correct. But having an 8-bit monitor means an available 256 colors.