Difference in the trajectory between inspiratory and the expiratory loop in the compliance curve is due to:

The difference in the inspiratory and expiratory component exists in compliance curve and it is called hysteresis The difference exists because greater trans pulmonary pressure is required to open a closed airway owing to deficit of the surfactants than to keep an open airway from closing, reflecting abundant surfactant Ref : Guyton and Hall textbook of medical physiology 12th edition, page number:319,320,321