Not sure why, but the UI seems to default to mirror striping for disks of equal size in Windows Server 2012 R2. In my case, 4TB + 4TB + 2TB (10TB total) created a virtual disk of 6TB, i.e. it seems two 4TB disks were used in RAID 1.
To solve this, you can use the
New-VirtualDisk PowerShell command, using the following parameters:
// this will create a "Simple" JBOD virtual disk over the entire storage pool
Make sure you remove any previous virtual disks from the pool. Also, you will need to refresh the Storage Pools page to see these changed after running the command.