In CiviCRM, there's a feature called "Smart Groups," which lets you automatically create a group of contact records based on a search criteria. Maybe it's a tag, maybe it's a certain type of contact, or maybe it's members from a certain region.
We recently had a client who wanted to know why the group count for the Smart Group had a different number than when he hit "edit search criteria" and performed a search for the same folks as were powering the Smart Group.
The important thing to note here is that despite a group being created as Smart Group based on search criteria, you can also always manually add people to a Smart Group ("this person should be in this group regardless of if they appear in search"), as well as remove them ("do not ever let this person into this group, even if they match the criteria").
You can see if someone is manually added to a Smart Group by going to that contact record's "Groups" tab. If they are in a Smart Group solely via search criteria, they will not appear here. If they are manually added, they will show here with a status of "Added." (Note that you will want to "Delete" the group membership here if you want this person's membership to the group to be determined by the search criteria - if you use "Remove" they they are put in the override category of Removed).
One final note - things change from time to time (people move, so maybe they should no longer belong to a particular regional Smart Group), so the counts for a Smart Group are calculated periodically and cached. Be sure you enable your "Update Smart Group Counts" cron job from the CiviCRM Scheduled Jobs configuration. You can also manually hit "update smart group counts" from the upper right, above the table of groups on "Manage Groups."