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.
Cited References
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.
After searching for your topic, on the results page below each article that has been cited will be a label: Cited by x. Click on that link to see articles that cite that article.
After searching for your topic, on the right of each article in the results will be a label: Times Cited: #. If the # is greater than 0, click the link to see articles that cite that article.
This index includes article information from approximately 8,500 journals in sciences, engineering, social sciences, arts and humanities (1900-current).