The issue is manufactures and OS developer.

Both see the size differently.

The short answer to there's two different measurement formats used. Decimal (GB) and binary (GiB) formats.
Binary is used by OS and decimal is used by the manufacturers.
Both the manufacturer and Windows are giving you the “correct” number.

Binary numbers are numbers that are a power of 2.
Decimal numbers are numbers that are a power of 10.

2^10 is 1,024 the closest Decimal number is 10^3 or 1,000
2^20 is 1,048,576 The closest Decimal number is 10^6 or 1,000,000
2^30 is 1,073,741,824 The closest Decimal number is 10^9 or 1,000,000,000

Detailed Information - Gigabyte