Q: Activate iPhone 6S

I've bought an iPhone 6S from a pown shop a few days ago. When I checked it for accounts, it didn't have icloud or any other account connected. So I connected to my icloud account and it worked. Except for one little big problem. The iPhone was unable to be restored to factory settings due to a security code which no one knew. So I decided to buy it and try to solve the problem home, with iTunes.

I connected the phone to my computer, to itunes, and set up a restore to factory settings. Now it asks me to activate it with the icloud account that was put in first.

I really don't understand how this works and why my iphone is still connected to another icloud account if when I bought it , it didn't have any account .

Anyone can help me?