In case anyone will still curious about this, I have an update.
We returned the phone today since we were able to secure a 128gb from the Apple store, and when speaking with BestBuy about my experience with buying an iphone from them, their explanation as to why the phone model given was not an AT&T Model A1778, and as to why the phone will lock to the carrier of the first sim inserted was this:
To make things easier for BestBuy this year, they only sell one model, the A1660, which is the one that has the CDMA and GSM bands, and are unlocked by default, so that they can activate them on any of the 3 carriers they deal with (AT&T, Verizon and Sprint). and not deal with having to stock carrier locked specific phones. the reason they lock upon inserting a sim was to comply with carrier requirements for AT&T and Sprint (Verizon phones by law cannot be locked) since 99.9% of the consumers who buy the iPhone from BB do so with starting new service or upgrades or adding a line in conjunction with financing the phone.
What was funny was several of the BB reps in both the Returns Dept and BB Mobile all feel that particular policy is absolute horse**** and agree that if you purchase a phone at full price, it should NOT lock to a carrier since they are unlocked to begin with. They also agreed that charging an extra $50-$100 more than Apple on phones purchased at full price is horse****.
So again, for me, lesson learned. I will ONLY purchase my iPhones directly from Apple, as I always have in the past.