The least mistakes of any of the ladies? What are you talking about? Her FS was a disaster. Along with the dramatic fall, she had an edge call on her lutz and under-rotated her 3X3 combo. Despite that she received the highest PCS for the FS and this allowed her place ahead of all the ladies and win the gold.
http://www.soniabianchetti.com/writings_scandinavium.html
I mean the overall competition, not just the FS. If we refer only to the FS, then Yuna should win obviously.But she made mistakes in the short and was placed fifth which makes it hard for her to win the gold even with a clean FS. If anyone was judged leniently at that competitions, I would say Costner. She made a lot of mistakes but was placed higher than Yuna. So if you are saying the judges propped up Mao's scores, I don't know why you don't mention Costner as well. As for higher PCS, I think it's because she has the most difficult programs (triple/triple and triple axel). Just like Yuna would have the highest PCS now scores since she has the most difficult programs.