Because your ad-buys are being submitted directly at the bid-ask feed auction, you’re subject to the real-time price movements on inventory, similar in concept to what you’d see in the stock market. The price of an impression is subject to the “ceiling” price by which you’re willing to bid-ask above the initial offering price, of which is set based upon the publishers “floor” ask-price, available supply and advertiser demand, within the ad-exchange.
TLDR: you set the CPM at the bid-feed level in which you’re willing to pay for an impression served.