The number of shots, like most all statistical applications, represents a bell curve.
The Xsi is rated for 100,000 shots. If you tested 1,000 XSi's, most would fail right around 100,000 with an equally proportionate but exponentially smaller number failing before and after this number until there are no survivors left.
Taking all of those images in short succession may have increased the shutter's expected life as well. The figure of 100,000 expects your camera to be several years old giving the parts inside time to become brittle, weak from being knocked around in your camera bag, ect.