Directly from this SO post:
Your comment to Hans answer indicates this is a COM assembly and that
you are using Visual Studio 2010.This means the assembly reference was likely added with the “Embed
Interop Types” setting set to true. This has the effect of linking the
COM assembly into your binary removing the need to deploy it
altogether. The following link has a more detailed explanation
•http://msdn.microsoft.com/en-us/library/dd409610.aspx If you do
want to deploy it though then will need to do the following •Click on
the reference in the references tab •Hit F4 to bring up the
properties grid •Set “Embed Interop Types” to False (this will ungray
Copy Local) •Set “Copy Local” to true
If you want to copy local, you must set Embed Interop Types to False for the assembly, in your case, Microsoft.Office.Interop.Excel. Embed Interop Types makes deployment easier for COM assemblies in that it embeds the assemblies within your assembly, taking away the need to deploy interop assemblies with the application.