I found this interview with her old drama teacher quite interesting (even though he's not an 'objective' witness (the bold bits are mine not the article)...
Mr Craige has also been to watch the show and was there this weekend agreed with John “Sam is awesome. She really blew everyone away with her performances, I can’t describe how electric her interpretation was. The judges were smiling and dancing along while she sang and they all gave her great responses afterwards.” Sam also got the new Oliver’s vote of approval this week and he was the second of the young boys to give her the thumbs up. When asked who he would like to see play Nancy, he replied with a great big smile “Samantha!” Mr Craige said “ All the young boys love her, she is so good with them and she has a natural, almost nurturing way with them. After the show they all come up to her and want to chat and be around her.” Sam is now down to the final seven and has proved herself to be a great, singer, dancer and actor. “It was really interesting to see the girls on live stage, as they will be in the production of Oliver, rather than live on the television.” said Mr Craige ‘What we discovered was that some girls had great stage presence and some next to none. When we watch them on the television the camera work provided for the routines can give some of the performers extra significance and va va voom, when in actual fact they have very little charisma when on stage. It works like that some times, some actors can look and sound great on screen but in the theatre carry no weight at all and visa versa. Samantha though is amazing and is as exciting on stage as she is on TV. The crowd in the studio went mad for her after her performance of “Since You’ve Been Gone”, a Kelly Clarkson song, on Saturday night, and she shone in the group numbers. She absolutely commands and owns the space she is in and fills her work with great emotion and energy. Its great to see but hard to imagine that this time last year she was studying for he GCSE’s in the Upper School Hall. But such is her talent, it really is a force of nature and she definitely deserves to win. She is absolutely right for the part in every respect and we just have to keep voting for her. Her journey and dignity with which she is conducting herself is inspiring and it proves to us all just what can be achieved when someone puts their mind to something they want to do and works very hard to achieve their goal.”
I've wondered about this, and whether the judges' sometimes bizarre assessments of some performers are because they quite literally experience something that the viewer at home can't: that all-important sizzle in the air.