The compressed resource is compressed actually by the RadScriptManager, but not from the RadCompression. The RadScriptManager compresses every embedded resource associated with it, so most probably the other script is not processed by the RadScriptManager by default.
The RadCompression though is not compressing JS, Css plus all *.axd handlers are skipped. It only compresses the AJAX requests by default, with an exception of the initial page load.
the Telerik team