In the real world, you know:
(1) all bids are submitted before being reviewed
(2) the highest bidder is actually going to pay the second price, and there's some friction in the form of a buyer's premium that makes it cost inefficient to immediately resell the item
Neither of these hold in the ad space.
Re (1), if you're trafficking ads through GAM, Google gets a chance to see the highest bid before deciding to commit to a bid.
Re (2), a publisher can insert fake line items that dynamically activate on a per-impression level. The goal of the fake line item is to be a stalking horse to artificially inflate the second-highest bid so that the second-price auction is effectively a first-price auction. If the publisher accidentally bids too high and wins the auction, they can just run the auction again.