The owner of the site may be able to charge a higher fixed fee, if the quality of the traffic is such that it delivers whatever the advertisers expect it to.
If an advertiser is buying strictly on traffic numbers, no pricing model can protect them. This is like saying that advertisers who buy on the most listened to radio station in the city are being defrauded. Advertisers who are ignorant to the buying tendencies of the audience for media will get screwed regardless. The idea behind fixed fees is that once the advertiser has made an informed choice, they pay only for the choice they made; they do not pay extra for things that can't properly be accounted for.