iPhone 6 asking for an unfamiliar Apple ID after restoring with itunes

Hi. So my sister bought this unused iPhone 6, 6-7 months ago. It was working properly until she changed the passcode and forgot it so we had to restore the iPhone 6 via itunes. After restoring it however, now during activation it asks for an Apple ID that wasn't the one set-up on the phone. I'm really frustrated because the Apple ID she has is supposed to be the first and only one that was linked to this phone. But it isn't that username. What is going on? 😕😟