How is the "Aggregate Committed Space" calculated in DFM/OpsMgr, and can it be adjusted?

The Aggregate Committed Space that I see in DFM is always seemingly too high, and this has been true in many version.  When I add up the volume committed space in the volumes contained, add the aggregate reserve space, and divide by the total aggregate space, the percent is always lower than what DFM says by about 5%.  Where do they get their number?  Am I missing something?

TIA,

Fred