Partnerships and Visibility of Systems

[Gorgo]

I don't understand this:
AFAIK, I can see every system someone I have a partnership with has ships, planets etc. in. Yet one guy ("B")I'm in a PBW game with keeps telling me he can see every system of mine, although he only has a trade and research alliance with me and regardless if a ship of one of his partners is in it or not. We have both partnerships with a third player ("C").

Is it some kind of chain thing, where he can look into every system C can? But then I can't explain why I can't do this reversely, because I can't look into those of his systems where C isn't in. fuse:

I hope this is not too confusing,

Gorgo