I don't know the calculations but in that same campaign Austria had the option and chose not to contest my PU over Brandenburg, so they do sometimes chose not to push it. I've also seen it a lot in some byz practice campaigns when I was trying for an early PU over Imereti; generally what would happen there is if the PU fires while I'm at peace Georgia has the option and declines to contest, but if I'm at war with the Ottomans Georgia'll declare.