That would make sense, but it doesn't appear accurate. Imagine a list of people, say on a computer spreadsheet. They're listed in the order they made the purchase, even though the service isn't being delivered then. So, say there are 50 people on the list (50 passengers paid for EBCI.) When it comes time for the computer to assign boarding positions, the first passenger to purchase will get the first number, second person gets second number...
Now the tenth person in the virtual line/on the list cancels a week before the flight. Passenger 51 then buys EBCI - either after the ticket purchase, or as part of a new ticket. Passenger ten's name was erased from the list/line, but the line wasn't deleted. There's an empty spot. Passenger 51 is inserted into that spot.