I think Tim was forced to play 4/5 for several years while there were sub-standard 5's on the team, or playing 4/5 with other players who could do the same (Dice, and to a certain extent Bonner).
With Splitter in the wings, I thing Duncan is trying to return to his natural position, even if it means temporarily conceding the 2003 arrangement with Rose ahem Blair filling the paint for stretches. This may be good news as far as protecting Tim from injuries from opponents in the paint, but it does mean more movement, which may result in more aerobic wear-and-tear. Just my 2c.