May be they are using some technology or algorithm to find that, but main concern is not from where they get result, more important is what can we do to decrease this ratio.....i dont know in what respect it effects but this matter is noticeable.....because google first looks main index and if it did not find matching query then it goes to supplemental index that makes searchability low......So i am trying to get this ratio lowered so that more pages of my site come in searchability....