Interesting observation! I have three theories to this.
The first and most likely being elves, similar to us humans, come in all sorts of heights.
Second theory is that perhaps the source collar wasn't Sebille's first. Perhaps her old Master preferred posture collars, giving her a bit of a stretch.
The third, I cannot say without spoiling her story line, but it ties back to her origins and perhaps that is of a separate clan.
