Some clients (In my experience especially Apple computers, and I think it is even in their documentation) REALLY prefer wider channels, so if you are running "auto" on channel-width, and one AP is at fx. 40Mhz and the other one is at 20Mhz , then the client might prefer the AP with the 40Mhz channel, even though it is far away. I always recommend "static" channel-width, so 20,40,80 depending on your local regulatory domain, and whats feasible.