Overly simplistic example: eBay bids $1 max and Amazon bids $2 max. People searching for eBay have a 10% probability of clicking on an eBay ad and a 2% probability of clicking on an Amazon ad.
Expected value to google: Ebay - $1 * 10% = $0.10 EV max Amazon - $2 * 2% = $0.04 EV max
Ebay is displayed even with a lower bid because the expected value of the display is higher. The bid is never adjusted -- eBay is specific about its bid or its maximum bid and the market of expected value decides who is shown. Amazon could just as easily bid $5.01 and show up. And when the ad is clicked, if Amazon is the only bidder at $2, then Ebay would pay $.50 for that click because that is the lowest bid that wins the position using the EV.
To apply this to the Snapchat Conversation -- One might expect if Snapchat operated this way, then its probably fair to assume that EveryTown would have had a much higher relevancy score than NRA, so should have been able to win the campaign for much lower $$. But in paid placement like this, everyone is paying for placement not for performance. It is not an EV market. Hence the dilemma.