There's no way of knowing for sure, but I'd say they're educated guesses based on reports and the time of year. Well, there is one way to know for sure: I pulled a tagged fish out of the action Thursday, so if it was tagged two days prior on Scorton Ledge, then we can be certain
(actually it was a pretty old tag, so that won't be happening). Seriously though, this week there was some dynamite fishing out on Scorton's ledge, lots of fish in the 30-34" with the occasional bigger bass, then the canal blew up Thursday (maybe Wednesday, but I wasn't there so I can't say) with fish of the same size class. So I think that's a pretty good assumption.
Also, given the time of year, figuring these fish are on their way south isn't too much of a stretch either, especially with that wind we had Thursday night.
When that tremendous push of big fish came through the fish week of June, right when the Canal was slowing up, I started getting reports of boats in CC Bay slaying good bass that were loaded with macks. I'd bet the farm that was the same body of fish from the Canal following that big school of Mackerel.
Fishing's all about trying to figure out what the fish are doing and where they're going to be, and the best we can do is try to guess.