This means that if you are a publisher, for example, the middlemen between you and the advertiser has economic incentive to report lower traffic, impressions, number of conversions, etc. This way they would have to pay you less and keep a larger portion of the revenue. The only way to get that data is from the middleman, and you have practically no way of verifying the legitimacy of that data. In a decentralized and transparent system such as AdEx, this is not possible — since the data is uploaded from every individual user directly to IPFS (another decentralized system designed for storage). The finality of all that data is settled between the publisher and the advertiser on the blockchain (Ethereum), in a way that incentivises both parties to report all the data they have accurately. This achieves a transparent bidding process between publishers and advertisers directly, without any mediators, therefore removing the ability of anyone between the publisher/advertiser, or themselves, to hide data and cheat the other parties.