If you have more than one pixel / stitch the software has more difficulty deciding which pixel has the dominant color. It may or not be a shading issue and can actually confuse things. Also, you want to keep the number of colors as small as is reasonable to minimize the number of inevitable confetti stitches. I don't know about Standard, but with Pro you can limit (by percentage) how many of the colors can be used for the background. That also limits confetti stitches.