Because the values that get mapped are the bits of the URL that matched a particular response descriptor.  

The idea is that you could map the data at http://example.com/things/12345 to an object with an ID of 12345 even if that ID didn't appear in the JSON.

The values that end up in the mapped object aren't not coming from the response itself, but from the mapping process; so I think it's appropriate to use `@metadata.mapping`.  I used the word 'arguments' because that's what the RKPathMatcher calls them.  And I used the response descriptor key path so that you can specify which descriptor you're mapping the arguments from.