The stats on ad impressions come from a report by comScore, which tracked the big video sites and ad networks in the month of July. The report also included viewership data for the sites displaying the ads.
Google sites reached more than 143 million viewers in July, while Hulu only reached a bit more than 28 million. With those numbers in mind, it might seem odd that Google managed fewer ad impressions than Hulu, even though it delivered more videos to more users.
Dig a little deeper and it's easily explained. Remember that YouTube doesn't play video ads on all of its videos; regular user videos generally don't have ads, while brand channels often do. Also, longer Hulu clips frequently have multiple video ads.
YouTube relies more on non-video forms of advertising, like banners and AdSense, including ad bars that pop up during videos. Hulu has some display ads, but the majority of its advertising is served in the form of short video clips that play before a video begins or during commercial breaks.