Storage Spaces Showing Low Capacity And Uneven Disk Usage

I've just been migrating data to my new Storage Space on Windows 8. I had to do this in stages because I had a limited number of disks available to me. I had 2x 2TB disks in a nas box, and wanted to move them to a new server, and also added 2 more 2TB disks. Due to the nas being quite full, I started with a 4-disk pool consisting of 2x 2TB and 2x 500GB disks, and made a parity space.

My process was as follows:

Create parity space with the 2x 2TB and 2x 500GB disks
Copy all data from one of the disks in the nas to the space.
Remove 1x 500GB disk
Allow the space to repair
Add 1x 2TB disk from the nas into the pool
Copy the contents of the other nas disk to the space
Remove the 500GB and add the 2TB
Allow space to repair

The space is thinly provisioned at 4.85 TB which is what the GUI defaults to when I create a new parity space using all 4 disks. (I don't actually create the additional storage space). This is also what I calculated it to be (1.81*4*2/3).

Now the end result is very uneven disk usage and a low capacity warning, despite having what I think is about 57% total utilisation!

Here is a link to a screenshot of the space's setup:

[URL]

1) why it thinks there is low capacity?

2) How can I rebalance/redistribute the data evenly over the 4 disks?