That has to be a metering issue. How many focus points do you have enabled? With spot meter, it may just be grabbing something else.

I do not have these problems on my cameras, but, every camera make meters differently. The light meter in your camera is what tells it to set the aperture or shutter speed or both, depending on the mode you are in. If I were having these problems, I would shoot manually so I could have control of the DOF and the exposure. At least over things like you have pictured in the last few posts. Where you KNOW where the subject will be.

Though, your dark images are not unsuable. They just need post processing to bring them back up. You paint from them so it really doesn't matter. If you were selling them, I would rather have the background blurred and fix them in PS(or just shoot manually! )

The histogram on your images do show that they are underexposed by a half stop or more. Here is the dog jumping photo brought up in PS