One of the earliest search engines, Yahoo, originally let other companies do the searching and it incorporated the results into its web site. Now it does its own searching. It has algorithms to determine if a link is a natural citation or not.
What is a natural citation?
Consider that some web sites may include words/tags in the SEO to permit it to be indexed into many categories, even if the website is not about that. For example, a website may be about French cooking, but the indexing tags include Spanish cooking, Chinese cuisine, Southern, and so on. These other tags are not natural citations, unless they clearly mention these other cuisines in the website.
Yahoo also includes paid content, skewing the results towards commercial interests, and ranking them higher than others.
Yahoo also tends to rank inbound links differently. When you perform a link check on Yahoo and then on Google, the Google results almost always tend to be lower. Google shows a snapshot of the “relevant" links, unlike Yahoo which shows all of the links regardless of relevance.