I think there was a change in the last few years about how they are appointed.
Previously: CO's meeting with elders consider a candidate and the CO makes a recommendation to the WT branch. The candidate is appointed after receiving the approval from the branch.
Now: CO's meeting with elders consider a candidate and the CO makes the final decision and appointment. The branch is not part of the decision. The servant is appointed almost immediately.
Someone correct me if I am wrong, but I think this change is pretty recent (in the last few years)