All behavior names for objects must be unique. So, as your object had reserved name 'pathfinding' already, and you gave it another pathfinding behavior through the family, C2 had to name the behavior as 'pathfinding2'. So your object effectively has 2 pathfinding behaviors. :)
If there was no renaming, you could not distinguish between the two behaviors. Like, if you did line like: 'Object.pathfinding.nodeat()', which path finding behavior would it refer to? The one on the family, or one on the object itself? :)