Why is the actual capacity of an SSD smaller than the labeled capacity?

This is due to system-reserved space (reserved space and over-provisioning).

Reserved space: The SSD reserves a portion of its capacity for firmware or controller management tasks, such as bad block management, enhancing durability, and wear leveling.

Over-provisioning: Some space is allocated to improve the SSD's performance and lifespan.

These are some of the reasons why the actual capacity of an SSD is less than the advertised capacity.