Is there any reason for a HTTP server to sometimes respond with Vary: *, and sometimes with Vary: Foo, to requests for the same resource?
What should a cache do, if after receiving (and caching) both responses, it then receives a request with a matching Foo header, for which the Vary: Foo response is suitable? Can it serve the matching response, or does the separate Vary: * response override it?
Each response is evaluated in isolation, so it can select the Vary: Foo response.
See: