groupofnames stores its members in the member attribute (using DN as the value)
groupofuniquenames stores its members in the uniquemember attribute (again using DN as value).
The uniquemember attribute however is designed to be able to hold an extra unique identifier to tell the difference between two DN’s who have the same value in a group. The reason why this might happen is that a user is deleted from the directory, but not from all of the groups. Later a new entry is added with the same DN, but it is a different person. This person needs access to the group, but you need a way to differentiate between this recent addition and the earlier DN (if you have several thousand members, simply deleting the earlier DN may not be a reasonable option).