Here's why:
First, remember that the DNS system that converts the IP address of your server into an alphabetic name is actually it's own network. If one DNS server doesn't know how to translate a particular domain, it asks another one, and so on until the correct IP address is returned.
The .com registry has "root" servers. Basically, a place that lists each domain. Prior to September of this year, these root servers updated themselves in batches every few hours, not immediately.
Now, the .com registry adds records dynamically and thus, a domain name becomes operational immediately.
The caveat is that some 3rd party registrars (usually the cheap ones) still process their DNS records in batches to save on processing and bandwidth costs.
I hope that this helps,
Rob from
Sinxstar Hosting
By the way, if you would like more hosting topics like this, please sign up for my newsletter at
http://www.sinxstar.com/