[authorize(roles="Admin"] v/s [authorize(users="Admin"]

Hello,
 
You have to understand tha a "Role" is kind of like a group of users (as in Active Directory), and that "User" is jus the user as itself.
 
As an example, you can have a user named "Admin" who is the site administrator, then you have users being granted administrator privileges and in that case their username will be "someone" but they will have the "Admin" role and as such be granted access to the ressource.
 
I know that can be confusing, but you really have to understand that the diffenrece is being made between a username and a role (or group membership).
 

Hope my comments will help you.