well, obviously everyone has different sources for their questions. We all dont use only OG. I'd say everyone just throws in their question to either ask further explanation or just to give others the chance of practise. Most questions I read here are from OG, PR or the 1000s files.
Btw, it's no wonder you dont recognize any questions, unless you crunched already several books. The pool is quite large!
In addition to that I have seen some Kaplan800
questions, ETS paper based question and various questions from various other sources.