Why are storage mediums advertised at higher capacities than what they actually are?

It's Windows, not the SSD.

When Windows says the disk has 222GBs free, it means the disk has 222GiBs free. 222GiBs is approximately equal to 238GB.

222 * 1024 * 1024 * 1024 / 1000000000 = 238.37

Largely for historical reasons, operating systems traditionally report sizes in power of 2 units but use the nomenclature for power of 10 units.