When performing an installation, system DLLs with older versions are not updated to the newer versions contained within the distribution.

For example, a distribution that included the Microsoft C Runtime Library
Merge Module lays down an MSVCRT.DLL whose version is 6.1.8924.0 on a
system that does not already contain a copy of MSVCRT.DLL. But a system
which already contains version 6.00.8797.0 of the DLL (it happened to be a
Windows-95B system) does NOT have its copy updated during the installation
process.

Why would this be? How can I get it to work the way I would expect, which
is to update older DLLs?