Each article database that reports the articles that have cited another article are retrieving the information from the content within their own database. Google Scholar will usually have more articles listed under their "cited by x" label because the database has a wider reach. Some of these citations might be from books, technical reports, dissertations, and other types of documents in the Google Scholar database.
If you have found a particularly useful article, has another author cited that article in their reference list? Use Web of Science or Google Scholar to find out. This works best for “older” articles.