[Question] If A records for a domain take time to propagate, then why are subdomains available instantly after setting them?

For example, if I point my domain, let's say example.com, to address 1.2.3.4, then it can take a while for the change to take effect, up to 24 hours, yet when I make a subdomain, like sub.example.com, it's available instantly. Shouldn't it also take some time to propagate to other DNS servers?