MacBook Air storage shows less than 256 GB

The 256GB is the physical size of the drive before anything is put on it.

Then, a certain amount is reserved as "spare" since over time small parts of the drive can wear out or otherwise become unusable and these "spares" replace those bad sections and things continue as normal for a very long time. Without these "spares" a drive can become unusable much quicker. This is true with any drive whether it is an SSD or HDD. This is usually the largest portion that is unavailable for general use. How much area this is depends on the manufacturer, the drive, and how long they want it to last.

Then there are also areas assigned for use by the drive itself and the file system to keep track of everything on the drive.

So, because of these variables which can vary from manufacturer to manufacturer, it is common practice to advertise the base size of the drive (which is how the actual chips are purchased and assembled in the drive).

Where are you seeing this exactly? Don't forget that the operating system files will take up some space.

Also read the fine print on everything.

"1GB = 1 billion bytes and 1TB = 1 trillion bytes; actual formatted capacity less."

from Buy 13-inch MacBook Air - Apple

It's normal. Storage is binary so conversion is not exact. This has always been a thing with computers. The marketable amount is given in base 10. 256,000,000,000 Two Hundred and Fifty Six Billion Bytes is simply translated as 256GB, but since 1 kilo byte is 1024 bytes not 10, and 1 Mega byte is 1024 kilo bytes and 1 Giga byte is 1024 Mega Bytes, the amount is slightly less.

There are also other system partitions taking up space which reduces the available space.

The full drive should be 251GB when looking at it in Disk Utility.

245.11GB+524.3MB+5.37GB=251.0043 GB

Computer storage space always (since computers were invented) has a variation between what is advertised and what the system reports.