Simple supply vs. demand, there is a higher demand for Fort St HS because it's more well known whereas few people are aware of St George Girls HS (not to mention demand is lower relative to Fort St also because it's an all girls school). Also, for most of us, Kogarah is very far compared to Petersham...
Academic performance of the school is independent of its demand. St George Girls generally has stronger maths results than Fort St, but for English they are usually fairly similar...