The plot shows the two sets of Landau levels (red: longitudinal valley, green: oblique valleys). The Fermi level is calculated for an electron density of 1x10^17cm^-3.
Notice the different slope of the lowest Landau levels of the to different valleys: The oblique valleys, which have the small effective mass with respect to the electrical quantization have the largest cyclotron mass (smaller slope). The longitudinal valley, which has the large mass for the electrical quantization has the smallest cyclotron mass (largest slope in the Landau plot). Therefore all carriers can be transferred to the oblique valleys at sufficiently high magnetic fields in this particular magnetic field orientation.
The offset in the origin of the Landau ladders results from a strain induced energy shift between the longitudinal and the oblique valleys. The value depends on the sample history and is in the order of 0.5 .... 1.5 meV.