The image sensor in your camera only has 12 million pixels total (R + G + B). That’s only 4 million fully resolved pixels. The image processor in your camera interpolates this information and produce an image file with 12-million pixels, but it can never have a full 12-million pixels’ worth of resolution.
This isn’t just a marketing gimmick though. In reality the 12-megapixel sensor actually has 6 million green pixels, and 3 million each of red & green pixels. Green is in the middle of the visible spectrum, so that’s where the camera (and our eyes too) get most of the contrast. You can think of green pixels producing an outline of a shape, and red/blue pixels adding color to that shape. And also, because of the way these green pixels are arranged (Bayer filter pattern), if you save it as a 6-megapixel image file, you actually lose a bit of information. To preserve the full resolution, you want to save it as a 12-megapixel image.