"N only" will:
A: Deny any hardware N-capable device that (for some reason) tries to connect with a a,b,g protol. The N-capable device will see the router, but wont be able to connect beyond a handhake of protocol.
B: It wont be detected by an a,b,g-device that lacks N-capability by hardware, since a a,b,g-device, this is self-explanatory, basically because its physically impossible for a g-device to see a newer technology than what existed when g was state-of-the-art.
Now, you run inssider from a hardware N-capable device, so innsider sends all handshakes, including n. It gets a deny/disconnect-pack in return on a,b,g but lists it as a "capability".
A TRUE "N only" would only have N, and never be able to activate/support a,b,g by demand.
In short: To be able to have the ability to offer a,b,g on demand (if needed) and still DENY a,b,g for N-capable devices trying to connect with a,b,g-protocils it has to have a,b,g active to send a deny-handshake. To turn a,g,b completely OFF would require a separate hardware-chip for a,b,g with the ability to be turned off.
Just my two cents (allthough here in Norway we don't have any cents).