Initial actions:
I wanted to copy some files that were very close in total size to the USB stick memory size (<8G).
So I checked size of all files vs available space. A few more KB were needed for everything to fit, so I checked if the drive is completely blank.

Problem:
But when I checked the USB stick, it had 67MB used space, although it was completely empty (as in no hidden files or partitions). Formatting it while monitoring space usage with my own software, reveled that while formatting it NTFS via windows explorer, the system it does allocate all space and after a few mili-seconds a temporary file appears and disappears removing 67MB from the drive's space.
(7.48 became 7.41GB available).

System info:
Win 7 x32 running on notebook, no viruses, no antviruses, no 3rd party programs installed (just blank OS).

Fix:
What I did to have all space:
I switched from DevMng to optimize for performance and reformatted to FAT32, 64k cluster size.
That made the 67MB of windows spy-stuff gone and showed me an allocated space of only 64kb.
The difference was enough for my files to be fit and copied in one run.

So can anyone tell me what was that 67MB of system-hidden space ?

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Thank you for your answers but, as I mentioned initially, there is no hidden anything on the drive; no System information, no recycler or any type of file/folder, fake ones included (the new pseudo-folders W7 uses). I check this independently of windows and explorer, with personal FM that shows everything in a file structure, including the unseen even if 'show hidden' is on.

The space is somehow allocated outside the file system, probably directly into the allocation table.