Pending I-485 transferred from Local DO to National Office?

[nosce_te_ipsum 0]

My I-485 has been pending due to name check for 1 1/2 years. At my most recent infopass appointment at a local DO to inquire about the status of the application, the officer said that my file has been sent to the national office, which, in the officer's own words, means that my case has been either denied or approved--apparently there was no case status notes in the system on my file. Puzzled, I said that about about six weeks I also had an infopass appointment to check on the status of the applicaiton and specifically on my fingerprints that expired. At that appointment I was told that the application is pending due to name check and that my fingerprints would be re-taken only after name check is cleared before the application is approved. I was told nothing about my file being transferred to another office then. Upon hearing this story about the previous infopass appointment, the officer checked the system again and said: "Okay your name check still shows that it’s pending. Your file must have been sent to the national office by mistake. I will request for it to be sent back here and you should receive a letter in about 4-6 weeks about the status of the application." How is this to be interpreted? What are the typical circumstances and reasons that prompt a local USCIS office to send an I-485 applicant's file to the national office many months after the interview?