You may have come across some paid orders in your system where the Status equals Fulfilled and the Status Reason equals Paid, in addition to paid orders where the Status equals Fulfilled and the Status Reason equals Complete.
What's the difference between the two?
At the time of fulfilling the invoice, if the Order has any Products w/ Shippable = Yes, then the Order is changed to Fulfilled Paid. If it doesn't have any shippable Products on it, the status is changed to Fulfilled Complete.
This functionality exists so as to distinguish between an order that has been paid and an order that has been fully "fulfilled", which is to say, that any shippable products were shipped out.