You know I have done that same thing. It was finally pointed out to me but I didn't realize that some pens have a different size bushing for each end of the lower barrel. This was pointed out to me here on this site too. I really thanks all of you for your insight and wisdom here pointing out what is simple to you but the things that us new turners do without even knowing it!
Yes, I noticed that turning right down to the bushing for the lower barrel, once finished and put together, top side of lower barrel is a little below the bushing mark. I will measure that same bushing with others that I have. Thanks for the comments and thanks for looking.