Multi core CPU single thread behaviour, not achieving 100%

Question

As you can see from the attached image the CPU graph on my dual core machine is weirdly symmetrica

Answer 1

(Should have used comments for this, but I don't have enough rep!)

I encountered this situation more than once before, and always with an application conflict.

For example, it can happen when compiz is started twice, or when it is partially unintalled. I found it could also happen when reading DVDs with two different players at the same time.

In my experience, it's not normal behaviour: the computer slows down quite a lot,which is annoying when you're trying to fix it!