They thought they'd be able to make it when accepting
There's a number of factors influencing if you manage to be established in time to meet the operators limit.
The nice strong headwind an 4000' might last until 1500' helping you reduce, or it may die down just as you want to slow and extend the flaps, delaying the much needed drag.
An initial miscalculation of the energy of the aircraft may delay the neccesary early deployment of landing gear and flaps.