Why did NASA divide Expedition 13 into two parts?

Question

Up until ISS Expedition 12, all are single parts, but why did NASA divide Expedition 13 into two parts?

Answer 1

Up until Expedition 12, everyone on the station arrived and departed at the same time. Expedition 13 was the first one that did not occur. I believe this was in part due to spaceflight participant Anousheh Ansari visiting with the mission that brought the Expedition 14 crew up to the ISS. Thomas Reiter instead returned on the Space Shuttle, as it was then flying again (It had stopped after the Columbia issue in 2003).