One likely cause to it is because of how 16-bit color works. In short, 5 bits are used for red, or 32 possibilities. 6 are used for green as green appears very bright at the full 255 compared to the other two and gives 64 possibilities. The other 5 are for blue. Provided that the colors use an even-step technique, red and blue will step by 8's with 9's every 4th one. Green will step by 4's and skip every other 16th one. The color, for example, 128, 67, 191 would become 132, 69, 189. Try taking out your graphics program and have half the image as the first color and half of the second. If you use 32-bit color display setting for windows, you could, quite noticably, detect a difference between these two colors. Blue is the darkest appearing color, then red, magenta, green, cyan, and yellow at the brightest.
