Well 100 percent marks are a rare case .
above 90 marks can come in one test and in another only 57 may be result due to :
General knowledge
some specific knowledge
Math calculations , (we can make mistakes in it as well)
series questions (arithmetic/geometric)
English part , as once you are asked comprehension , another time its grammatical correctness , synonyms/antonyms
Even in our case NAO 1999 , some peoples only got questions from section 4 -9 , while other got from last sections (many candidates don't even read whole section)

Well the Game changer item from above is definately General Knowledge , related to Pakistan/Islam .I don't go in details of it, most of us have seen PPSC /FPSC tests and compare general Knowledge questions in all these tests