if you are using thin, there will be differences in provisionedSpace Size and usedSpace Size...on thick allocation, both will have the same values
When using snapsnots, the values will show the used size and plus the size of all disk deltas (as André said above)
about thin allocation:
not that this is a bad idea, the drawbacks on thin allocation lies at the more administrative overhead on your datastores. Some experts say thats a little bit of performance loss on thin allocation, which is a very discussable topic... you might want to find some articles about (try at Cormac Hogan blog. He's THE storage guy when the subject is virtualization).
and btw, dont think mr LucD Sleeps at all 