I think the reason you only see the # of matriculants and not acceptance is because most people who matriculate have been accepted at many schools.
I think the MSAR is keeping track of the number of individuals.
So for instance, if X number of matriculants had two acceptances thus the number of acceptances is double the number matriculants; however, there are still only X number of people with acceptances. Therefore knowing the number of acceptances and keeping track of them is not important. This is not the case for all situations, but I presume it is likely very close to the reality of the admissions situation.