It is expected that there may be differences between Sitecore Analytics and Google Analytics data. They use two different ways of gathering information. Google Analytics is a JavaScript-based client-side technology, while Sitecore Analytics is a .NET-based server side one, which typically means more accurate data.
Not all incoming requests are counted by Sitecore Analytics. It distinguishes unique users with ASP.NET sessions and browser cookies. Also, it skips robots' sessions. Thus, the number of unique visits in Sitecore Analytics is usually less than Google Analytics reports show.
Note: there is a known Issue about incorrect traffic type resolving.
Note: a significant difference between Google Analytics and Sitecore Analytics may indicate that xDB is not properly configured. The following article can be used to troubleshoot possible issues for Sitecore XP 8.0 or higher: Troubleshooting xDB data issues.