The Animus is gaming's greatest get-out clause. Invisible walls, erratic NPC behaviour, narrative inconsistencies and technical snafus can all be explained away by the fact that you're merely experiencing a (mostly) sophisticated virtual reality simulation. If guards are attacking you despite your notoriety being zero, well, that's just a bit of dodgy Animus programming. And if Niccoló Polo is refusing to follow you because he's busy hovering above a rock? Glitch in the Animus. Pedestrians merging into Ezio during a dialogue sequence? Animus.