For advertisers bidding on a maximum CPC basis, Quality Score on the Display Network is evaluated on the:
clickthrough rate (CTR) of all keywords across the account and relevance of the ad text and keywords to the advertiser’s landing page
click-through rate (CTR) of the ad and relevance of the ad text and keywords to the publisher’s site
Quality Score of all campaigns and regional targeting settings
Quality Score of keywords across all campaigns and all ad groups
Just like the Search Network, Quality Score is calculated for Display Network as well. But Display network is totally different animal. Google measures the relevance between ad text and keywords to the publisher’s site, not landing page.