Bushings not concentric, or a bent mandrel is the most cause of off center barrels. Also some bushings get 'skewed' when you tighten the nut. I have found that once I place the things on the mandrel, I will run it on low to look at the bushings. If they run out, I loosen the nut, twist things a little, tighten and check again. That can take out a lot of wiggle. Tail stock alignment is another issue, and will not cause this problem, in my opinion. If all other things are good, including the center 'setting' good into the end of the mandrel, your pens will be concentric. Other thing can be how you sand, and the grain of the wood having 'hard' spots on one side and softer on the other. Mandrel-less turning is an excellent way to get away from this problem.
