Using the Manage Sentences dialog to search for Playing (or any of the percentages) includes ignored sentences in the search results; a collection that is listed on the Dashboard as having 0 sentences playing can return a much larger number in the search, if the collection contains ignored sentences that have a percentage, be that anything between 0 and 100. This is illogical, as ignored sentences are by definition not in play.
Furthermore, sentences marked as Known are counted as Playing, both in the search and on the Dashboard. I find that illogical, too.