Great Article by Lisa Niemeyer on why media overdelivery isn’t always great

“Forecasting impressions in Digital can sometimes be the most challenging.  Not only might consumer attraction to a particular site or content be hard to predict, but the digital ecosystem allows for sourced traffic and dispersion throughout an Ad Network or Ad Exchange.  For endemic sites and share-of-voice objectives, inventory can fluctuate based on consumer interest.  For ad exchanges or programmatic buys with a quality goal such as percent in-view, the impressions tally may keep running up until the viewable impressions goal is achieved.  There may also be excess inventory the publishers will grant as goodwill/added value for the advertiser.”