I-485 interview in NSC

Thanks to xiaobaitu and all people who are very helpful here.
My I-485 (EB2) was transferred from TSC to NSC for standard interview, but I saw people here got the interview from their local offices, I'm in chicago now, why TSC didn't transfer to chicago for the interview? I'm still waiting for the interview notice, just feel very confused, hope someone can explain to me. again, Thank you very much!!!