The ones that aren't are called false drops. The larger the database, the more potential for false drops. Precision is defined as the ratio of relevant citations returned to the total number of citations returned. And herein lies the problem with Internet searching. It is very easy, using Internet search tools, to get high retrieval.
High precision is another matter.
Unless a searcher is well versed in the intricacies of search syntax of the various engines, high precision can be almost impossible to achieve. In addition, even if precision is achieved, the question of quality has not been addressed. Tools like Alta Vista and Yahoo are voluminous databases. They boast about that fact. But, as the above fable shows, they can be frustrating.