How does my xt1225 have Marshmallow?

I just picked up this phone on eBay from a seller in Puerto Rico. The phone has Marshmallow, which I didn't think much of it until I came here and saw people complaining about not getting updates on the phone. Got me wondering if my phone has an official update or if the previous owner flashed an "unofficial" ROM. Any way to tell?
Everything on the phone works great - this thing is a beast.

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