The observed and ab initio calculated propensity rules for collisions of NH3 with rare gas atoms are found to be in reasonable agreement for NH3-Ar, whereas for NH3-He they show large discrepancies, In order to examine these discrepancies we have calculated state-to-state integral cross sections for collisions of NH3 with He using the close coupling method. The NH3-He interaction potential has been obtained from SCF calculations, augmented by a multipole-expanded damped dispersion energy. Our calculations show that the discrepancies can be accounted for if the cross sections are corrected for the imperfect initial state preparation in the experiment. They also clarify why the discrepancies do not occur to the same extent for NH3-Ar. After comparing our new theoretical results with the experimental data we found that for one experimental cross section for NH3-He the earlier assignment must be corrected.