I got the numbers a little mixed up. The ATC numbers only show 512 completed or 20% which sounds more realistic. There seems to be a very large discrepancy between the ATC number and the ferry numbers. Maybe the ATC numbers are those who reported completing the thru hike and 400 have yet to do so?
Although I don't spend a lot of time in Gorham, I go there pretty often and didn't see unusually large numbers of hikers in town this summer or fall so I was pretty surprised by the tally in Maine.