It is a mystery and I'm also affected by this since my tinnitus changed in 2008, to variable tinnitus. It can be completely silent, mild, moderate or severe. I accept it because to dwell on it or try to figure out the reason for this can cause stress and affect you more. Try to take your mind off it and direct your thoughts to more positive things. I find this helps me a lot.
Thanks @MichaelLeigh I've been trying to do that lately. It's been getting easier and easier. Even though my tinnitus is worse than before my mind is stronger.
@Michael Leigh I have always compared my tinnitus to yours. Mine behaves in exactly the same way. Even today it was so quiet it was barely there even with plugged ears. But only an hour ago, I could here it over the tv. It's such a strange beast.