My position is this: They wanted a new "signature armour" for the game cover, and it had to be better than the T-51 while also looking like their "original" design, the T-45. But it also had to be in the prologue scenes as they were heavily featured in the pre-release footage, so it couldn't just be a BoS-improvement of the T-45 (which would have made a lot of sense).
But there was probably a lack of communication, so the loading screen featuring the T-51 was overlooked.
I feel that we can say, to a certainty, that this is exactly how it happened. I firmly believe that each and every game by Beth will feature "newer and better" things with little to no regard for lore.
The T-70 PA design! The latest design in PA technology deployed to the front-line 30 sec. before the bombs droped!