Printing on light garments is much easier than printing white on dark garments. On a light garment, not as much ink is required to achieve good coverage. Also many times you're working with thinner inks. Ink varies in viscosity according to how much pigment it has in it. For instance, a black ink is almost always going to be printed on lighter colors, the pigments needed for the black to show up are considerably less then the pigments needed for a white ink to cover a dark garment. Thus, you're black ink is much thinner and easier to work with. You can do almost every technique wrong when printing black ink on a white shirt, and there's still a good chance that your final print will look pretty good. Since the ink is thinner, it passes through the screen mesh much easier, with less force, and clears the screen better. (For those unfamiliar with the term clearing the screen, this defines the point when all ink is cleared out of the screen mesh and properly transferred to the shirt.)