Why is the youtube channel URL displayed as a Unique Identifier?

Question

YouTube

Good afternoon.
The question is, there is a channel on youtube, once I assigned a URL to it now, going to the channel, I see this:
channel/UCs2Mz-wwddC-nbTtK5Tk_MQ
Going to the Yotube settings-Advanced, there is no link to change the URL
But if you access the channel at the URL that I previously set for him, "Example youtube.com/user/test", he will go to my channel.
What's the catch? Channel linked to Google+