The data in the Group Information tab is not always up to date. Find the Last update at date to identify when the group was last refreshed. If that date is not recent, then the statistics in the Group Summary table will not be up-to-date either. To update those statistics:
  1. Navigate to Constituent360 > Groups
  2. Enter the name of the group in the search field
  3. Click Search
  4. Click on the name of the group (it's a blue hyperlink)
  5. On the Group Information tab, click Refresh Summary
You should encounter the message "The request to update group summary information has been submitted. Revisit the page to view the updated information." If the numbers do not update automatically, give it a few minutes (up to an hour if it's a big group) and then refresh your browser window.

For query based groups, check this solution.