(A link to a post I made regarding this phenomenon. Still don't know why it happens, but the post shows where the reported number seems to derive from.)
EDIT to say: Mine didn't go away (firmware v4.3.4) - I use Win7 x64.
Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.0.2 Mobile/9A5259f Safari/6533.18.5)
My 32gb usually shows 54.6gb available is u look at it in windows explorer
Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.0.2 Mobile/9A5259f Safari/6533.18.5)
My 32gb usually shows 54.6gb available is u look at it in windows explorer