The reason is simple, the Billing systems and the back-end OneCare operations servers are entirely separate systems, probably contained in entirely different hosting locations. The reason for this is simple, the Billing systems require a different level of security and contain a completely different type of data than most others.
It's likely that the updates from the Billing systems to OneCare occur as a batch process run at times when both systems have less operational overhead, so this may only occur once a day.
In truth, there should be no real difference shown by OneCare when using a PIN, since these are also entered into the billing system. In fact, I believe I recall such a delay when I did this myself earlier this year. I do know that at least some parts of the Billing system didn't display the 'new' expiration date until after the previous subscription had expired, since that's how the system is designed to work.
Tieing the Billing system directly into every operational system such as OneCare would be highly complex and actually dangerous to your personal credit card and other data, so personally I'm glad they've kept these systems separate.
OneCareBear
