Second hand iPad activation problem

I have bought a second hand iPad and have been using it for 6 months. My icloud account is associated to the device, and have reseted and updated it several times.

Today I reset it again, and although in the initial screen it says "the owner of this ipad is" and appears my Apple ID, when the activation screen shows up, it says "Ipad locked to owner" and the Apple ID is completely different (I assume it is from original owner).

This is the first time it happens to me, and can't tell what is happening, since the device was icloud-free when I bought it, and was associated to my account from that moment on.