Files left after formatting
I can think of two relevant facts.


Apps that were pre-installed as part of the system (i.e. came with the phone) are not deleted by a factory reset: they just go back to the original version, and will probably be updated by Google Play later.
If you had Back up my data turned on in the device settings (on the Backup &amp; reset page), then as soon as you signed into your Google account, the phone will have started to reinstall all the apps you had installed previously, and the data of all the apps that support Google backup.