Comment 4 for bug 571018

Disk space is allocated in megabyte-sized chunks for performance reasons.

That implies that there will be sub-megabyte areas of unused space between partition table and partitions and at the end of disk as well. Similar to the partition tables, raid and encryption both have metadata which imply further alignment requirements.

For whatever it is worth, I find those "unusable" things in partman to be illustrative, but yes they can be confusing for the novice user. As such, we could consider showing them only when the installer was started in "expert mode".

<offtopic extension="note">
Would (even single-disk) partitionable raid1 by default make sense? I find such rather useful for VMs that can be booted both on bare HW and in VM, since raid1->gpt->data (inside VM) is about as easy for the bootloaders as gpt->raid1->gpt->data (on bare HW).
</offtopic>