Virtual Machine "Provisioned Space" and "Used Space"

I am trying to get the "Provisioned Space" and "Used Space" for a Virtual Machine, as reported by the vSphere Web Client in the following screenshot:

I am using the VirtualMachineStorageSummary data object (vSphere Documentation Center)

I am collecting the committed and uncommitted properties, and then doing this:

spaceProvisioned = committed + uncommitted

spaceUsed = committed

However, I see differences in the numbers reported by the API vs the numbers vSphere web client shows.  Most of the virtual machines numbers reported by the API match the vSphere web client, but some are a few % off?

e.g. for the machine JK-IMPACT, I get spaceProvisioned = 44.11GB, spaceUsed = 38.83 from the API

but, in the vSphere UI (as shown below) it is: 44.11GB and 35.2GB !

So the question is, am I pulling the wrong properties from the API, or is there something else I am missing in the way that the vSphere web client uses to calculate/display Provisioned and Used space.

Cheers,

Ian