Why doesn't the divider go all the way to the top of the ceiling of the hutch?

The dividers are to separate the sleeping area with the rest of the hutch.

Although there should be some barrier between these to areas it's important that there is still enough airflow for your pets when they're using the sleeping compartment.

Although there is an opening that your pets can use to gain entrance and exit with a large amount of bedding (recommended) the entrance can get blocked.

So having a gap at the top of the divider is important.

The height of the divider and the gap that's left is of no risk to your pets and doesn't affect the ability of the divider to create a sleeping space.