It's not a typical tracking number and it's not designed to provide "up to the minute", accurate, updates. It's just access to the USPS informed visibility system which has been around forever and usually in the realm of high volume mailers. eBay's providing the service at cost... other companies would charge you extra per envelope.
There is usually... always... about a 2 or 3 day lag between the time the number indicates delivery and the day that it will actually arrive at your buyers mailbox. That's because it's not actually scanned at the final, local, destination post office... that happens at the closest sorting station. So if it says delivered today, add two days and that's about the time your buyer will receive it.
