If I understand the question correctly, I think you are asking “what happens if you select two tag names in the tags list of the sidebar”. This is where I think that talking about boolean “and” and “or” operations gets confusing. What you get by selecting two tag names is a list of items that have either tag A, or tag B, or both tags A and B.
Another way to explain it is like this: the set of items shown by DEVONthink is the union of the set of items tagged with A and the set of items tagged with B.
Note that in your example of tags named A and B (each with two items), the union of those sets may have fewer than 4 items, if some of those items are tagged with both A and B. For example, if I create two documents and tag them both with A and B, then
- the count of items with tag A will be 2
- the count of items with tag B will be 2
- the count of items with either A or B will also be 2