hey Gorik! yeah this is an admittedly confusing feature and is something we've thought about changing.
Basically there are 4 types of relationships possible between two glossary terms. You can have a "HasA" relationship, both outgoing and incoming. You can also have an "IsA" relationship, again both outgoing and incoming.
In this situation, you are setting Term A contains Term B which means that Term A "HasA" Term B - outgoing for Term A and incoming for Term B.
We don't actually show the incoming relationships in the UI for any term, just their outgoing ones related to other terms. That's why you'll see Term B under Term A's contains, but not Term A in Term B's inherits. The Terms in Term B's inherits page are really the outgoing "IsA" relationships for Term B, not the incoming "HasA" relationship.
I think it would either make sense to simplify this model or provide a way to see all possible relationship types in the UI to avoid this confusion,