Total capacity of Rotating Magnetic drives used to be as stated. In addition to all the numbered blocks into which you could actually write data, there were quite a few 'spare' blocks, that the drive controller kept in reserve to substitute for any blocks that were found to be bad, a few at the factory, and many more in the field.
With SSD drives, in some cases the TOTAL capacity is stated, and because blocks are dynamically re-numbered, a small amount, typically 5 to ten percent, are held in reserve as spares. These are not directly available for holding data.
Marketers have taken great liberties with stated drive sizes since there were drives. The classical disclaimer used to be "formatted size will be smaller".
I do not think there is anything wrong with your Mac.