Visitors not Converting Goals
If your site spans multiple domains or has subdomains, then the tracking code must be set up correctly in order for your conversions to be attributed to an AdWords campaign. The customizations needed are described in Tracking Multiple Domains.
If a goal URL or funnel step is for a page that is on a different domain or subdomain, and the necessary customizations are missing from the tracking code, your AdWords users do not convert towards your goals. Since the referral information is not be applied to a page on a different domain or subdomain that doesn't have correct tracking code, the session is also counted twice for the same user because a new session is started when the user goes to a different domain or subdomain.