My only guess would be that the flight number range is intended to distinguish "AlaskaSkyWest" flights from AS codeshares on American Eagle flights operated by SkyWest (or, in the past, Delta Connection operated by SkyWest). It looks like they only allow online connections to Alaska prime flights (which makes sense; they don't want to deal with the added complexity and risk of handing an unaccompanied minor off from AS and their policies to a partner airline and their policies unless they've done work to make procedures compatible). Was there some time in the past when AS 3440-3499 covered all the AlaskaSkyWest flights and they used that old list?