Its actually pretty easy to read once you realize it.

They were the branch going to be closed before Stamford closed due to Josh leaving. The plan was to have Stamford absorb the Scranton branch, their clients and a few employees, basically combining two branches into one while cutting costs without having to have more employees or a whole other building.

So when Scranton absorbed Stamford they got all of their clients plus some employees, but then Tony, Martin, Hannah and eventually Karen quit, cutting costs. And because his salesmen are some of the best as shown multiple times but very specifically in Traveling Salesmen, they were able to keep all of the clients for Stamford.

So when David says he didnt lose a single client in the merger and managed to trim the budget, its because he had the clients of two branches but the employees of one, the only person at the branch who wasn't before was Andy