I doubt that recreating the control helped mje but perhaps he will let us know. In my experience, using the well-know scientific trial & error methodology, the number of columns in a tree is to a large extent dependent on how many are visible in the GUI. If you delete a column you need to resize the remaining columns to fit in the control or reduce the control width, making sure that your last wanted column boundary is just hidden beneath the control border. Then adjust the number of columns property.
Be warned: if you later drag the right-hand border of the control to enlarge the real estate you will magically get that pesky extra column again - the number of columns property will increase by however many columns you expose!
It seems to me that a better behaviour would be to just make the extra space to the right the number of columns dead, and never alter the number of columns until the user explicitly sets it.