Let's take another perspective. Take for instance that 6 yr FO at AA you were referring to. Say that same FO left TWA six years ago when he was one number junior to the 10-15yr FO at TWA. So the 6yr/FO came to American, went thru probation, sat in crash pads and years of "B" scale/low wages and has paid his dues at Amercian.
All the while his buddy stayed at TWA, held a line, no crash pad/reserves and had decent wages.
Now does anyone think that the FO that stayed at TWA deserves to come over at American and be senior to the 6yr/Fo that made all the sacrifices.
Ladies and Gentlemen, seniority is the years of service at a "SPECIFIC" company........not just any company.