Hello Michael,
The way it is interpreted is the following:
Add a card to the filter deck "if its deck is A or its deck is B".
This makes reasonable sense, if you think it as an expression that applies to individual cards.
Since a card can have a single deck, add card to the filtered deck "if its deck is A and its deck is B" would never match any cards.
But I understand that if you think of it as "add all cards from deck A and from deck B" then it makes sense, but that's not way it works.
The reason it is done this way is that it is actually simpler to express filter this way. Otherwise something like: deck:A is:new added:2 would either need a special semantics or would add all cards from deck A and all cards that are new (regardless of the deck) and all cards added in the last two days (regardless of the deck or as to whether they are new). A shorthand syntax like "deck:(A and B)" would reasonably make sense, but then "and" would mean different things in different contexts, which would be confusing for some other people.
The Anki manual has some examples of "and" and "or" under the searching section.
I hope this helps clarifying things.
Flavio