Samsung SSDs conform to the International Disk Drive Equipment and Materials (IDEMA) standard, which classifies approximately 93 percent of their actual physical memory capacity as usable space for storage.
To make calculations easier for consumers, manufactures rate drive capacity based on the assumption that 1 GB is equal to 1,000 Megabytes (MB) rather than 1,024 MB. When calculating capacity, the OS uses the more accurate 1,024MB, resulting in the discrepancy you notice when looking at the reported capacity versus the actual capacity.