And it has more than 256? Because I count about 4.
Rather than being condescending without actually explaining anything, I'll explain anything.
Saving the image in a 256-color format will cause you to lose color information. You don't pick which 256 colors go into the image; you get a small set predetermined by the format, and colors will be changed to approximate one of the 256 colors. Therefore, if the colors he's got in the image were ever so closely related but ever so subtly different, they wouldn't be anymore after conversion to a 256-color format. They are now the same color.
Low-grade 256 colour .gifs are basically a legacy of the old SLOW INTERNETS ARE SLOW days. Any file saved in that format is 'optimized for web display', meaning whatever colours are found in your orginal image are converted to their closest analog on that narrow 256 colour spectrum. The gag is that regardless of whatever colour differences may have existed originally in that image, Maddox's choice to save them as a low-grade file has eliminated any such distinction.
I HAVE NOW EXPLAINED THE JOKE, EFFECTIVELY KILLING IT.
SIR, WE HAD TO BOMB THE VILLAGE TO SAVE IT.