FamilySearch uses volunteer indexers and a two level system.  The same data is indexed by two different people.  The two results are compared, word  by word.  Any difference is highlighted and a third person takes another look before making the final decision. FamilySearch volunteers come from many parts of the world.  They can choose what records to index and are likely to select records where they have the most familiarity.  (I have  done some indexing for FamilySearch)
I believe Ancestry employs paid indexers, many from outside the U.S.A.  I don't believe their indexing is subject to the same level of scrutiny as FamilySearch

David Rosen
Bosto, MA