The information in the "Group Information" tab is not always up to date. Find the "Last update at" date. If that date is not recent, then the statistics in the "Group Summary" table on the same tab. To update those statistics, click the "Refresh Summary" link to queue up a task to update the information. That takes usually takes a few minutes but can take up to a few hours in some situations.

For query based groups, check this solution.