Very good question that I had to ask myself. Had I not turned them I could enjoy not knowing. It actually started by accident when I had to turn them on one of the valves to get the wrenches out. After turning them I checked the valve clearance again to make sure and found it had changed. I then started messing around with turning the nuts after tightening them and found this problem. Maybe it's not a problem and I have decided to live with it until my next valve adjustment. I did go through and readjusted all the valves (practice makes for speedy work) and left them at that without messing with them. I'll check the valves on my next adjustment and see if anything has changed.
Rusty