The seat availability isn't cumulative. It shows the number of tickets the airline will sell in that fare bucket, but the same seat may be counted more than once.

For example, Y5 B5 doesn't mean that there are 10 seats. It means that there are 5 seats that they will sell in either Y or B class.

Also, the availability is capped, so the tools won't show more than a certain number no matter how many seats are available. It's usually 7 or 9 (varies by airline), but maybe NW uses 5?

Finally, there are still a few people who don't get seat assignments at time of ticketing, so they would not show up on the seat map - that's why occasionally you see more empty seats than availability.