Why is my citation count on Scopus lower than on Google Scholar

Google Scholar is a search engine and hence has a much wider coverage than Scopus, including theses and unpublished materials, etc. These are content types that are not indexed on Scopus and therefore will not contribute to the citation count so therefore the Scopus count is likely to be lower. On the other end, the citations that are covered by Scopus have a higher scientific impact than the ones that are in Google Scholar but not in Scopus.