Compellent SC4020 Size Question

Hi

I've hopefully a very simple question that someone will be able to answer quickly, but it's been niggling at me for a while and, even after attending a week's course on the storage center, I still can't figure it out.

In Enterprise Manager, on the Storage tab for one of our SC4020s when it is selected in the tree, there are a few space values reported. I understand (or at least I think I understand) what all of these values are referring to, except for one - "Allocated Space". I first thought that this was the total of the configured volumes on the device, but it's not - it's less. For example, the device has a total Available Space of 12.55TB. It's over-provisioned in that the total configured space for all of the volumes comes to 15.2TB, which is fine. However, this Allocated Space figure is 8.95TB, which doesn't seem to relate to anything - and as far as I can see, this is the only place that that figure is actually reported.

Can anyone shed any light as to what this figure is / means?

Thanks,

David