Different pool of dungeons, changing the set up for affixes, in addition to the normal change in difficulty are no reason?
Maybe so those who do the content get a shot at tier before the “catalyst is open, 4 piece tier or bust” takes over and is used as reason to deny people spots in groups or refuse to do content with them.
In a vacuum these things might be bad or make little sense, but nothing Blizzard does is ever in a vacuum.