fly4more
As my colleagues have said, rightly or wrongly that's a facet of how the BA system works ( another gem is having stacks of seniority but being stuck on a dying fleet). Even if, for whatever reason, you can't change fleet you're ahead on seniority and will be above the later joiners when you do eventually switch.
I'd look on the bright side - at least these days with the likes of pprune you can make yourself aware of the potential pitfalls before signing on the line.