I don't get it, for an exclusive game, none of this should be happening.
4 things could have happened:
1: the engine is crap and is hampered by performance issues, well at least for the way that UBI has implemented the engine for this game.
2: This game was initially designed to be multiplatform, but when exclusivity was negotiated they continued work with the same engine and didn't bother to cater to the 360's strengths by creating a new engine.
3: This game is going to the PS3 as the 360 only has timed exclusivity, so to have parity with the unreal engine which we all know runs like ass on the PS3, they were forced to make specific sacrifices on a technical level to achieve their goal.
4: UBI programmer don't know squat and have managed to brake a perfectly working engine that's well suited to the 360 hardware.
Personally I thinks it's a little bit of each of the 4 options.