Sean Gallardy explains a limitation of sys.dm_hadr_cluster_members:
I’ve now run across a few different instances where the monitoring for quorum was done via this DMV. On the surface, it seems like nothing would be wrong with using the “number_of_quorum_votes” column to check on the members of the cluster and see their voting status. However, this isn’t quite the case… you see there are various mechanisms that influence whether or not a member (or witness) has a vote and these continue to be expanded in each version of WSFC.
Click through for a short history lesson as well as some good advice on how accurately to get this information.