There are two issues here. One, at this point in the competition, it's about the body of work, not just the last performance. Carly and Syesha have been very inconsistent. One performance isn't going to suddenly give them a fan base.
Two, it's not just about the pipes! It's about the total package of performance. Carly and Syesha can sing very well, but they don't know how to use the instruments they were given.
Brooke may not be the vocally loudest or widest range, but she's had a lot of good performances prior to the last few weeks. Her voice has a unique quality to it, and she puts a lot of emotion into her performances. She won't win, but I think she's the third most talented. She proves you don't have to bellow every song to have good and interesting performances.
As for Jason, well, I don't get him, but a lot of people seem to connect to his performances. I can't explain him. :)