When you think of the guys that San Ant has started over the years at the center position its easy to see it as more of formality. Guys like Kurt Thomas, Nazr Mohammed, that dude Oberto, Rasho Nesterovic etc.. Pop had those guys there to take pressure off Duncan by letting them guard the other teams big guy.
I think Duncan had his reasons for staying listed as a PF after Robinson retired. Him coming in as a rook w Robinson as the HOF center/cap on the team already is one big reason he started off his career being listed at that PF position.
I think he stayed at the position for a few reasons:
1. The problematic All Star situation. There was another center by the name of O'neal who landed there at the 5 annually. It would make it difficult for him to start every yr if he listed himself as a 5.
2. I also think he wanted to defy the stigma of the plodding bigman with limited skills, ie inside game only.. TD could/can step out & shoot that little banker off glass from angle, hit 20 ft jumpers consistently, handle well and have the offense run through him completely, all in addition to his dominant low-post game.
3. Lastly, imo he likes for people to think of him as the GOAT at the PF. He can't claim that if listed as a 5, no way.
Make no mistake tho he is a center.