Up to 100 top domains will be shown if you are a subscriber, or for legacy credits users, if you have at least 1 credit in your account (this is a free service that will not affect any quotas, credits or limits on your account). You can learn more about our subscriptions plans or sign up to one here.
Sometimes large domains might have multiple IPs - our system currently takes only one into account. If we detect such cases we will recommend searching for another IP.
Sometimes a clearly bogus domain name can be shown in the list (example) - it can happen due to bogus DNS resolves. Since results are ordered by highest number of referring domains it is usually not important, but it is always wise to keep an open mind about such cases.