Should not "Clear Data and Clear Cash" clear also user sign in credentials? (DROPBOX)

I have a samsung S3 Neo, normal not rooted.

I wanted to clear Drobox data from the device and I noticed that when I go in the Application Manager and when I do:
- Stop the app
- Clear data
- Clear cash

the user credentials data is not deleted. When I click on the Dropbox icon (like that I want to open the app), it automatically logs in again without asking me user credentials.

How is this possible?

When I do those 3 steps for any other app that requires login (Facebook, Twitter....), the user details are also deleted, and when I start the app I need to sign in again.

If it can log in again, where does dropbox then stores the user credentials? Seems like they are automatically retrieved from somewhere.

Thanks
Cheers