The PAM matrices are considered nonreciprocal, meaning that the probability of changing an amino acid such as alanine to arginine is not equal to the probability of changing an arginine to an alanine. Why?
PAM matrices are considered as non reciprocal because these point accepted mutation are influenced by the natural selection. It means that these mutation are necessary for the survival and advantageous for the individual. This lead to evolution and due to the evolution and better chances of survival after PAM decrease the chances of its reciprocal conditions.