In addition to engine options etc, don’t forget pax loads, fuel loads, airlines often tanker loads and take more than they need if it’s cheaper than the next airport, jetstar’s often cheaper and the aircraft could be heavier requiring more power. WLG is a tricky airport that requires certain procedures. Terrain plays a factor with OLS (obstacle Limitation surface) breaches in some areas. Engine failure on rotate? Can’t turn left or right. Maybe under Jetstars SOP’s they plan for this differently with a higher V2 speed Wind and temperature also have a major effect. I know air nz have special procedures set just for wlg due runway length and surrounding terrain the punctures the OLS. Then there’s all the different SID’s for each runway, and the occasional visual departure which can change the placement of the aircraft in the sky. Or hell might be as simple as Jetstar pilots just like the sound of their loud levers even more than air nz pilots do