There are a few reasons this can happen:
  • If the group has not been rebuilt recently, the statistics for the group may not match the query results.  To fix this:
    1. Find and view the associated the group.
    2. Go to the "Group Information" tab.
    3. Click the "Rebuild Group" link. Once you click the link it may be a few minutes before the rebuild is done.
  • If you run a Mail Merge, the record count in the Mail Merge results may be less than either the query or group because deceased records are filtered out of Mail Merges.
  • If any of the constituents in your query are in the "Site Administrators" group, they will be filtered out when the group is created from the query.
For non-query based groups, check this solution.