At least it makes some sense, as they are mostly based on ISO 3166, as well as:
the international vehicle code for South Africa has been “ZA” since 1936. ZAR serves as the ISO 4217 currency code for the South African rand. South African aircraft registration prefixes also start with Z.
South Africa is .za from Zuid-Africa, the dutch term for the country
At least it makes some sense, as they are mostly based on ISO 3166, as well as:
SA is the country code for Saudi Arabia.