This is usually do to a corrupt transaction.  I recommend you run a verification and rebuild.  If that does not fix it, you'll need to do some investigative work to find the corrupt transaction.  In my client's case, it was a corrupt credit memo that totaled $39.99, yet QB was seeing it for customer balance total purposes as $1,239.99.  Once I deleted this credit memo and recreated it, my customer balance total in the customer center matched the customer's true open balance.