Customers that have already been exported continue to show up in the list of customers to be exported.
When changes to a customer are saved the customer has been exported flag can be cleared so that the customer and it's changes will be re-exported.
This behavior can be changed in Application Settings.
If you are viewing customer details and don't make any changes - click Cancel; You should only click OK if you make changes to one or more of the fields. This will prevent accidental clearing of the exported flag.