Why does my USB drive show less capacity than stated on my PC or MAC

The difference is due to way that most storage manufacturers and PC manufacturers define a scale. There are two systems decimal and binary.

Storage devices use a decimal system and PCs use the binary system -this leads to some discrepancy between actual capacity and reported capacity by a PC. The difference can be between about 2% – 9% depending on the capacity of the drive.

Some additional capacity is taken up by features required by the operating system e.g. FAT tables and the flash drive controller e.g. spare blocks for wear levelling.

Decimal (Dec) bytes Binary (Bin) bytes Ratio (Dec/Bin) % Diff
Kilobyte 103                               1,000 210                                       1,024 0.9765625 2%
Megabyte 106                       1,000,000 220                                1,048,576 0.953674316 5%
Gigabyte 109               1,000,000,000 230                       1,073,741,824 0.931322575 7%
Terabyte 1012 1,000,000,000,000 240               1,099,511,627,776 0.909494702 9%