This is amazing that you find out that in their system you were still GC holder, I was thinking about this too, because when I got my certificate, it was kind of wet paper feeling. After the Oath someones photo fall off from their certificate too. So they print certificate may be a day before Oath, and right before Oath they take your GC and Oath letter with Questionnaire on the back, so this is your final interview. and if everything goes well they give you Certificate.
Someone lost their GC before their interview, so they let him take interview, he has sent copy of GC with application, but when they collect GC before Oath, they make him fill a form. So I believe after Oath the same day or next day they enter in their system that you are a USC. I hope this make sense.