Most likely they produce a flash chip with an actual 16 GiB. But to reliably use that chip in a USB flash drive, the flash controller needs both: 1) some spar

Most likely they produce a flash chip with an actual 16 GiB. But to reliably use that chip in a USB flash drive, the flash controller needs both: 1) some spare blocks to use to replace blocks that fail during usage. 2) some amount of space to store its own data (spare block map, in use block map, failed block map, etc.) and the controller just uses part of the existing flash chip for those purposes. The difference between 'module' and 'actual' is likely "controller data overhead".