SOLVED MySQL usernames with no underscore?

Hello

I´m using MariaDB 10.2.17, centOS 7 and Cloudlinux 7.5
I don´t know if this is something from MariaDB or CL.

The point is...

I just added a new account in one of my servers, and created a new database and user.
Always the user and dabatase name uses the cpanel account main username, like :
username_database
username_username

But in this new site i added, when i created a new database:
database
username
(without the username_ before)

Is right or i have something bad in the config?
I can´t understand how it works now.. if site1.com create a new database called "db" and site2.com in same server created a new database called "db" ???

Thanks for your comments.
Fabian