I've been digging around in the database and the web trying to determine how this numbering system works.
I see that sales_flat_invoice.increment_id has the following information:
`increment_id` varchar(50) DEFAULT NULL COMMENT 'Increment Id',
It is stored as a string and has no auto_increment value applied to it. I'm trying to determine how Magento really determines what value goes into this field. There were some articles stating that the value comes from eav_entity_store.increment_last_id but that doesn't line up with my current numbering sequence. increment_last_id matches my current orders but does not match the most recent sfi.increment_id.