Indeed, 1000 and 1024. But is this not just OpenWebIF but also the OpenPLI own menu system. This could explain the small difference (I think) between both of them.
The problem being raised in this post is a different one. It is that over a period of time, most of the free capacity of the storage medium disappears.
As others have noted in this thread a reboot and as I noted here also a system file check, restores the proper "free" capacity value.
So maybe this sounds more like an deeper issue...
Its possible that the difference reported of capacity used between the OpenWebIF and OpenPLI UI menu's is caused by the 1000 vs 1024 difference.
But apart from these (small) differences, both report a comparable steady and gradual loss of free capacity. Would 1000 versus 1024 account for that?
The question might be how is the (available and used) capacity data reported to OpenWebIF and the OpenPLI UI menu's, by ? (kernel?)