This is because of the units used. Advertising/sales uses units with factors of 1000 (KB, MB, TB), while computers use factors of 1024 (KiB, MiB, TiB). So your

This is because of the units used. Advertising/sales uses units with factors of 1000 (KB, MB, TB), while computers use factors of 1024 (KiB, MiB, TiB). So your 1TB is definitely 1000GB. But it's 931 GiB. Some claim it was easier for consumers to understand factors of 1000 rather than 1024. But conveniently it also makes the number look bigger for no extra cost, so eh. I'd put my money on the later.