I'm not sure, but, all I know is, if this were being launched on the PS2 or Xbox, I'd expect the sales to be doubled... maybe tripled. Plus it'd be hyped to death.
The GCN usually doesn't get or have much hype.
However, this year has been a very good year for the 'Cube, and I wouldn't be surprised if RE4 put GCN sales back on the map.