Can someone explain this?

I recently installed Windows 2000. The first update I installed was
Service Pack 4, and the next one was Update Rollup 1. After that,
Windows Update said that I still needed to install Update Rollup 1,
even though it appeared in my Add/Remove Programs. I went ahead and
installed all of the remaining updates except for Update Rollup 1, and
when I went into Windows Update again, it still said that l needed to
install Update Rollup 1. I ran MBSA 1.2.1, and that reported that I
was missing 2 critical updates: 823559 and 891781. Both of those are
among the updates included with Update Rollup 1, so they must not have
been installed for some reason. I went ahead and re-installed Update
Rollup 1, and finally both Windows Update and MBSA were satisfied.

My question is: why would the Update Rollup 1 fail to completely
install the first time? It makes me wonder if anything else I've
installed has only been partially installed.