b/c in the RFC that describes server linking, if the whois request has a specified server then it is sent to all servers, but only the server that the specified nick is connected to is allowed to return a result

AFAIK, this is b/c if UserA sends a /whois request for UserB, who's 2 hops away, then:
1) UserA doesn't have to know what server UserB is using
2) it guarantees that the server UserB is on get's the request
To clarify things when a server is specifed: the whois request is sent through that chain of linked servers until the server userB is on replies(at which that server doesn't continue sending to it's links)