<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Of course they don't say _how_ NW made room for more of those seats! (Or more precisely, how they squeezed more in).</font>
It has been reported elsewhere (in FT, I think) that on some NW planes First class seat pitch is as low as 34". Given that on AA all Y seats have pitches ranging from 34"-38", I'm not surprised they could add that many seats.
Geez, maybe they just bought up all the old AA seats and stuck them in F.