The balance showing at the bottom of the record is a cache balance that will not necessarily match up with the balance on the Activity tab. If payments or charges have been deleted, this amount might show differently than the actual record balance.  Scheduled Activity, not yet generated, can also affect the balance.

The balance on the Activity tab is the balance that will print on statements and reflects the current balance for the record.

If the cached balance does not update after a reasonable amount of time (within 1 business day), take the following steps: 
  1. In Plug-Ins, click Update Cached Record Balances 
  2. Mark the Run plug-in checkbox 
  3. Click Update  
Or, try adding a new charge to the record.  Do not post the charge.  Then delete the new charge - this may trigger a refresh of the record balance.