Since both Opportunity and Curiosity are on the surface of Mars how can their present distance from Earth differ by 6+ mio. km? And even with Mars Express in or

Since both Opportunity and Curiosity are on the surface of Mars how can their present distance from Earth differ by 6+ mio. km? And even with Mars Express in orbit it doesn't appear right that it should be 20+ mio km closer to Earth. I wonder if these were the distances relevant when th probes were launched?