See Atlassian Answers Content By Label discussion.

OR logic between labels makes sense as long as you don't have any required labels, i.e. it will return pages that have at least one of the labels specified. However, once you add a required label (+label), the other non-required labels become optional and the results include pages with just the required label. The required label should be applied in addition to the other labels listed in the configuration.

Operator: OR (default)

Configuration: aa,bb
Results: Returns pages labeled with aa, bb, aa/bb

Configuration: aa,bb,+cc
Results: Returns pages labeled with aa/cc, bb/cc, aa/bb/cc, and cc (<-- this, pages with just cc, is the problem)