Wife had 'combo' interview and both I751 and N400 were approved same time. In fact they actually never interviewed her for the I751. The IO only asked is that your husband sitting over there and she said yes and that was that. He typed some things on the computer and moved straight to N400. While I was sitting in sitting area I was getting real time updates in USCIS tracking as the IO was updating the system. It was so surreal. Haha she had her naturalization cert (same day) before official I751 approval showed up in the mail a week later. I always say she had a combo approval not interview. 😀
So combo interview is definitely a thing but it's not always the outcome as in op's case and yes sometimes field offices play a role.
What I often recommend here is file N400 when you are eligible and hope for a combo interview. If combo doesn't happen then on the plus side you've already gotten the ball rolling on N400. So yes combo interview is a thing but NOT always the outcome.