The discrepancy between the advertised capacity of a storage device, such as a 32 GB SD card, and the actual available space you see on your computer is due to the difference in how storage is measured by manufacturers and operating systems.

Storage capacity is typically advertised using decimal-based calculations, where 1 gigabyte (GB) is defined as 1,000 megabytes (MB). However, operating systems use binary-based calculations, where 1 gigabyte is 1,024 megabytes. This means that when you buy a 32 GB SD card, the manufacturer is using the decimal system, and the actual usable space you see on your computer is calculated using the binary system.

So, when you convert 32 gigabytes (in the decimal system) to the binary-based system used by your computer, the actual available space is slightly less. Additionally, some space on the SD card may be reserved for the file system and other system-related functions, further reducing the usable capacity.

The calculation is as follows:

$32\text{\hspace{0.17em}}\text{GB}\times \left(\frac{1\text{\hspace{0.17em}}\text{GB}}{1,000\text{\hspace{0.17em}}\text{MB}}\right)\times \left(\frac{1,024\text{\hspace{0.17em}}\text{MB}}{1\text{\hspace{0.17em}}\text{GB}}\right)\approx 29.7\text{\hspace{0.17em}}\text{GB}$

This difference in measurement units and the reserved space explain why your 32 GB SD card appears as 29.7 GB when viewed on your computer.