"Balance team tests the fights themseslves"

[Jaquan]

They know all the mechanics and intended strategies before even stepping in and, as they've said before, do it in max ilvl gear rather than what normal people would have like crafted sets.

I would imagine they have a pretty easy time of things

Actually if the posts before it is accurate it's the opposite. The only advantage they initially have are wipe prevention mechanics which allow them to learn the fight at faster pace (less restarts). Afterwards they need to clear it with min ilvl needed for the instance and without wipe prevention.