In Mac OS X, in any given major version, (10.4, 10.5, 10.6
) there will be a series of sub-versions (10.6.0, 10.6.1, 10.6.2
).
When Apple releases a software update, they actually send out two kinds of packages. One is an update that covers the changes from one minor version to the next (10.6.3 to 10.6.4), and wont work for two or three steps (10.6.2 to 10.6.4 = cannot).
The other kind is a smart update, or Combo Update. This package is larger, but can update your computer from any version to the current version, as long as it doesnt cross a major version boundary. So it could update 10.6.1 to 10.6.4 in one go (but not 10.5.x to 10.6.x). They always send out combo updates in the standard Software Update application, since its more universal.