User blog comment:D. Cello/Assassinews 03/05 -- ACWiki's Fan Section and AC3 first trailer/@comment-96.235.189.37-20120307030715/@comment-1888015-20120307184641

Pre-rendered cutscenes have a lot more detail in them and take extensive amounts of time to produce. If you compare a pre-rendered trailer to this one, you can tell it is made with the game's graphics as pposed to being pre-rendered.

Recently, Ubisoft have updated their Anvil engine, from what they call Anvil 1.0 to now being Anvil 2.0.