High confinement and stability are obtained simultaneously in stationary conditions in improved H-mode discharges at ASDEX Upgrade. The improved H-mode discharges are typically composed of two different phases: ‘lower heating phase’, where H98(y, 2) is similar to standard H-modes (H98(y, 2) ∼ 1), and ‘fully developed improved H-mode phase’, whereH98(y, 2) is higher than standard H-modes (H98(y, 2) up to 1.4). In this paper, the confinement physics is studied by comparing these two different phases using ASTRA simulations. The results are compared with experimental observations. Firstly, the time evolution of the q-profile is simulated with ASTRA using experimental kinetic profiles and compared with experimental measurements. Secondly, the two different phases are compared by power balance analyses and predictive modelling using the Weiland model with ASTRA. Ion temperature gradient lengths and turbulence levels measured by reflectometry are compared between the two phases. Lastly, the roles of density peaking and pedestal pressure in confinement improvement are discussed. The transport analyses using the ASTRA code and experimental observations show that the fully developed improved H-mode phase is very similar to the lower heating phase in terms of core heat transport. With respect to the confinement improvement, it is suggested that the increase of the edge pedestal pressure plays an important role. Enhancement of the radial electric field at the edge is believed to be closely linked with confinement improvement at the edge pedestal region in the fully developed phase.