Wow.... there are different people on different teams for different purposes...
You think they have the guy that balances numbers go over and create some verdant spheres one day? Seriously?
Maybe the guy that is creating a new boss mechanic is told to stop and go draw some soul shards...
Blizzard surely only hires people that are completely skilled in every possible aspect of the creation of this game, just so they can do this!
that'd be a looooooooong resume!