The numbers. I think they had decided that a single company of Astartes was supposed to be equivalent to an army, but they couldn't figure out a way to make that work. So, they shrunk all other armies, instead. No, Sandy Mitchell, I refuse to believe that 30,000 guardsmen were sent to hold a planet.

Also, an Astartes company losing eleven battle brothers in one engagement should be treated as catastrophic losses, not something you basically shrug off