I'm sorry, I'm not debating that different amounts of reps can effect the types of gains experienced, but do you really think 2 more sets with a little less weight are going to totally restrict any strength gain while still upping mass? the muscle is going to respond to all the stuff you throw at it, its not in a vacuum.
yes more fibers mean that a muscle can work at a given weight for longer, by spreading ( more moving around) the load. if it raises it's ' max weight ' to its going to have an easier time moving a lesser weight too. simplistic way of putting things but I think it gets the point across.