This is manufacturing related condition and can be caused by the dye shifting in the same sheet of carpet. One edge of the carpet can differ in color than the other. If this sheet is installed in one room without a seam, the color variation from one edge to the other will not be noticed. However, when the carpet is seamed, the light edge is connected to the dark edge. This causes a color variation at the seam.
Color side match can be corrected by feather blending. Feather blending is performed onsite by applying carpet dyes to the lighter side of the carpet to blend the color.