The conceptual difference between users in a standalone site and a network site is that in a network the users do not belong to the site but to the network itself are are being automatically logged in into all the network site once they login to one. Most of the time the difference can be ignored but there are some edge cases in which the differences come to like, like trying to make an anonymous comment when you are logged in to the network.
In a network you don’t add users to a site but only to the network, then you give them permission to do some actions on the site they were registered for. In a standalone registration implies automatically some permissions.
This implies that as a subsite admin in a network you can not modify a user as it is not the sites user but the network’s one. The most that can usually be done is to change the users permissions for that subsite.