We used to always fight as our GC so the achievements only count to our current GC.

The problem was mostly people choose Limsa as their GC so victories were skewed in their favor and it was all imbalanced. So they created a merc feature and then decided to force it to happen always. But the achievements still base on your current GC and it may or may not take some spaghetti code untangling to make it more logical to how the system works now.