Adjustment of Status - I-485

I had an interview in beginning of may.. Didnt have a lot of proofs .. Some of them, but not as many as the most of people do.. we are young, students, without a lot of belongings.. we live at my wife's sister place.. Got separated at the interview, didnt went perfect as it should've went. We were not that good prepared bcs we live together and we have nothing to hide, so i taught it will be so easy..
for 2 months and more after interview, i didnt get anything, no info, no mail, nothing.. Such a hard time, finding a peace..
After a bit more than 2 months, immigration finally came to our place, we were not home.. But they know we live with my wife sister, and she was there. They interviewed her, that went perfect as it could've because she knows us, we live together for like a year now.. So i taught thats a good sign, i should get approved next day.. Its been 8 days since they paid me a visit.. I started being nervous again.. Like what else do they want, why they didnt approve it. And than i called USCIS customer service(which is not very usefull, all info they give u, u can check ur self on case status/processing time etc) but ive asked to transfer me to officer so they can provide me with more info...
The guy said im here in LA (there where i live and where i had an interview) and he said my case is going thru background check..
My questions
Why background check now, didnt they already do it when i did fingerprints???
How long it takes to go thru that?
Is that final stage, could that mean my case is already approved but they want to make background check before?
Anyone had similar info?
Do they maybe say background check just to get rid of me and thousand other people that are calling them every day or ?