I cannot tell exactly why this is happening, perhaps there is somekind of container where calculation go wrong (most likely you are running this report on the World object???). However - you seem to have a semi large environment. You can change the policy a little bit to show better results (which did at our site).
Click at the "View Information" the "Change Default Policy Configuration". This should give you a pop-up. Go to point 3a, deselect "Disk Space -> Datastore, Containers" in the "by Allocation" column. The by Allocation column is more conservative. If both by Demand and by Allocation are selected the most restrictive will apply. Which is the Container at by Allocation selection. When checking on cluster level, this should be fine, but a rollup of multiple containers (host into clusters into datacenter into world) will generate 'wrong' results.
