I think they look at the player's total votes, then put the players with the most votes on the teams if they can credibly put them at an available position. Which means that if KG gets some center votes and some forward votes, they'll be combined, and if he finishes 4th among centers but 6th among forwards, he'll be a Third Team forward. If he finishes 7th among forwards but 3rd among centers, he'll be a Third Team center. Guys like Pau, KG and Duncan could be thrown in either position.
Yes, they combine all forwards, so First Team will be Durant and Lebron, Second Team will be Love and (undeservingly) Griffin. But wide open after that between guys like Garnett, Aldridge, Pau, Dirk, etc. I think Tyson Chandler will take Third Team center.