It is possible the microbiota in dogs is better able to resist colonization of pathogenic microorganisms than the microbiota in humans.
In other words, the resident "bugs" in a dog's gut may be more resistant to bad "bugs" (like salmonella) colonizing in a dog's gut as compared to the resident "bugs" in a human's gut. The resident bacteria in your gut resists bad bacteria from setting up shop.
Interaction of Salmonella spp. with the Intestinal Microbiota