One thing lead to another, (as they do), and I starting thinking about the way the center drill makes a slight jump as it enters the work. Maybe this thing is not that well aligned. I couldn't remember how anal I had gone about aligning the thing back when I did it in 2004.
The lathe quite accurately between centers with taper of only a few tenths over 3 inchs, so it can not be far out.
I set about measuring.