I have seen this many times. Usually it happens when they have already assigned all of the seats that are available for advance assignment. That doesn't mean the flight is full (although overbooking is also a possibility). It may just mean that the only empty seats are either (a) exit rows and bulkheads, which are usually available only at check-in, or (b) premium seats, which are only available to elite frequent fliers. All of these become available at check-in, though, so you could actually end up with an excellent seat. Just be sure to check in early (on NW
you can do it from home up to 30 hours before your flight--very convenient!).
Common sense is not so common. -Voltaire