Short Answer: What Kimsland Said
Long Answer: Unlike hard drives which are generally larger and have the size artificially capped (ie: 80GB platter capped as 40GB) by the manufacture to whatever size they want. NAND memory is sold in fixed capacities ie: 8Gbit (1GByte, 1024MB), so you start with 1024MB, take away space for the controllers firmware, spare blocks, translation tables, and whatever proprietary modules / data structures are unique to that controller or manufacture and that gives you the raw space logical space. Then you throw the filesystem on and that adds extra overhead. All this means that one manufactures 1GB flash drive may be bigger or smaller than the others depending on how its configured. The only time you’ll see 1024MB in a 1GB flash drive is if it’s a fake (the controller has been programmed to lie about its size) or if they stuck a 2GB flash chip in the drive and capped it at 1024MB.
In the latter I’ve seen that twice, on cheap Chinese thumb drives given out at conventions, the memory chips were from the reject pile (failed tests, to many bad blocks) and the extra 1GB area was used as spare space for remapping bad blocks.