I'm not sure how the system is working yet.
Yesterday, I flew coach msy-IAH on a 24 seat 757. I am Plat and was on a not-so-cheap "V" fare. I checked in seq #2.
Earlier in the day, I checked my wife in for IAH-PHL today. She is Plat and was on a very low "X" fare. She was seq #1 and was assigned the last FC seat on a 733 when I checked her in.