Adjustment of Status - I-485

I just had my interview today with my husband. My PD is oct 20, 2017. Check my other posts if you want to know about my other timeline.

We arrived half an hour early. After checking in and everything, we were called into the office without waiting too long.

The whole interview was pretty light and relatively short, at least shorter than I expected. The IO mostly asked our names, where we live, our birthdays, who proposed and how did that go, stuff like that. Nothing was intense. The IO even called us cute couple at one point. Toward the end she asked me lots of yes or no questions, just the ones on the i485 forms.
Then the IO stepped out of the office for like 5 minutes (it sounds like the IO went to take care of some issue not related to our case but we are not sure), came back to us and said they will do a background check, which should be quick.
And this is the part I got confused. At first the IO said our case will be approved, as soon as the background check pass, which is pretty quick, and she will try to finish it by today. But before we left and I asked how long should we expect the result, the IO said there's the 120 timeframe thing, but I should hear back in about a month, A MONTH!
SO WHAT DOES IT MEAN???
My case status is changed to "Interview Was Completed And My Case Must Be Reviewed" a few hours later. The email explains the next step to be "USCIS initiates the background checks". And I should be expecting a biometric appointment, which I've already done. I'm very sure that none of us has any criminal or arrest or charged history whatsoever.
Did anyone else experience this? Is it normal?

I'm just so tired of Uscis's unclear answers and timelines and just want to get it over with. Hope the rest of my fellow applicants would have better experiences.