Note how similar the tone is on the black on black and white on white photos -that's exactly what your camera is designed to do, expose the photo to make the scene average 18% grey.
The black on white and white on black photos are exposed closer to normal because the high contrast colors together make a more "average" tone. Still, though, it doesn't get it quite perfect, does it?
How can you overcome these problems when you're shooting similar scenes, but want them to come out right?