Why are not all assemblies in GAC built as MSIL?

.net,gac,gac-32,gac-64,gac-msil

When you compile your library you can choose to target either "Any CPU" or a specific processor architecture. The "Any CPU" libraries only need a single entry in the GAC and the entire assembly is compiled to MSIL. Other assemblies need a different library for each architecture. These libraries are...