So far I understood that the hyperfine field energy is intrinsic property of the nuclear magnetism induced by the electron cloud, which is constant value for the state in question.
What is the reasoning for the changing intensities observed in the pictures should be the decrease in Doppler broadening due to the decreased temperature, thus decreased energy distribution among the ground state population in atoms. This degenerates the energy states and shows the intensity of each peak according to the transition probability.