Main Office v. Main Branch

On my way to work this morning, I saw this notice plastered on the glass window of a bank.

This main office branch will soon be open to serve you better.

I was bothered by the phrase "main office branch" in that sentence. Isn't it that the main office is not a branch. That is THE main office.

I uttered the same phrase a long time ago, and I remember that my father corrected me on this one.

The correct expression should be "main office:"

This main office will soon be open to serve you better.