My suspicion would be that they realized it wasn't going to be a very big shift from the previous games and that the differences would more so be "different" than "different and better", and modeled it off of similar "filler games".Rex called me out on that language too. You're right... it did meet stated expectations and for all we know it could end up being highly profitable.
I was using the term "struggle" more in terms of what similar titles in the genre have done historically. Benchmark FPS releases in that quality score range have been bigger sellers in the past. Maybe its a reflection of a new market for FPS games, maybe there's more impact from digital on this title than others. Tough to say.
But why would a company have expectations that low when similar titles have been so much bigger? The question then becomes why that assumption was settled on; whether that change in expectation was market driven or driven by something they saw in the title itself is a good question.
Like some examples that come to mind:
Gears of War 3 -> Gears of War: Judgment
God of War 3 -> God of War: Ascension
Borderlands 2 -> Borderlands: The Pre-Sequel
Batman: Arkham City -> Batman: Arkham Origins
Assassin's Creed 2 -> Assassin's Creed: Brotherhood/Revelations
Like all of those titles saw notable sales declines versus the previous entries to my knowledge, but the publishers would all expect much larger sales for the next mainline entries (Gear of War 4, Borderlands 3, Assassin's Creed 3 which did 12.5 million, God of War 4, and Batman: Arkham Knight).