The app could detect me even after factory reset. How?

[masterton]

I have factory reset my OS and install the app. However the app knew I used it before.
How could the app manage to detect me even I have reset the system?
What method(s) does the app use?
How could I prevent it from detecting me?
Thank you.

 

[B. Diddy]

It may have backed up user data to your Google account, assuming you had your Google Backup turned on.