Where Does FamilySearch Find Their Documents?

I have the scan of my Great-Grandmother's birth certificate I got off of Family Search, but to get an official copy the archive says they don't have it. Szukajwarchiwach.gov.pl shows they haven't scanned the volumes for that year (and many years around it). My question is, if the archive can't find it - how did familysearch find it? If I go to the archives in person, can I search the books myself? If not, would there be any value in going in person? My Polish is not that great, but I can get by - and I can write out my situation and all the details I know - so I'm not worried about a language barrier.

Could there be a situation where FamilySearch has a different source of documents than the archive?

I'm pretty sure there are different genealogical databases. Just like when you try to find a person online - many services and they somehow work with different dataset because you won't find the same information in all of them. FS could be more commercialized and pay to use other data that is larger than szukajwarchiwach.gov.pl