Has the Adv Search changed in recent months or years? I seem to remember clicking more than one + box, and it acted like an OR filter (more boxes gave more results, unless you hit the - box to filter stuff out.) Now it looks like it works like you would expect: more + boxes narrows the results, rather than increasing them.
Unless I remember incorrectly.
It seems like the sections act as AND and choices within a section act as OR, with oddness around the excludes.
But it is better than where I was before...