After the preparations were complete, Hellsend assaulted the 10th floor. It wasn't just one zone, it was an entire floor. There were also numerous problems that arose. Some of them were unexpected.
First, while there were 28 zones on the 6th floor, only about 13 of them fell. North America was a battlefront that took the balanced approach. At any given time, only 17 of the zones per floor would be cleared.
This was done to allow the new reapers to "train" on the 11 zones that fell daily. I say daily simply because these zones had no defenders. About a week after the zone was recovered, it will have its Soul Crystal corrupted again and begin to summon the undead.
Since souls were the basis of the reaper economy, it was necessary to provide people with a means of earning it. This meant that for the 6th level. Only 2 zones fell by accident, while 11 of them were created by design.