Manual WiFi Channel Selection Is Incorrect / Ignored

[#1]

Hello All,
A simple problem: When I select a channel in the Wifi settings of my 2704n router, it is completely ignored. For example, if I select 4 or 9, it insists on using 11. See attached picture from Wifi Analyzer.

Just in case, I've tried IE and Firefox. Not that that should make a difference.
Anybody know what might be happening here?
Thank you.

1 - Product name: Plusnet Router
2 - Serial number: N7151031B002032
3 - Firmware version: 7.275.2_F2704N_Plusnet
4 - Board version: F@ST2704N
5 - Mode ADSL
6 - Uptime: 22 days 09:39:23
7 - Data rate: 928/12915
8 - Maximum data rate: 1192/13148
9 - Noise margin: 5.9/3.2
10 - Line attenuation: 12.8/24.5
12 - Data sent/received: 2.1G/911.1M
20 - Broadband username:
21 - Wi-Fi network/SSID: PLUSNET-8M6R9H
22 - Wi-Fi connections: Enable(802.11 b/g/n(up to 300 Mb/s))20 MHz WPS enabled
23 - Wi-Fi security: WPA2
24 - Wi-Fi channel: 9
25 - Firewall: Default
26 - MAC Address: 00:37:b7:bf:17:91
27 - Software variant Version 1.0
28 - Boot loader: 7.275.2

[#2]

That's what happens in "up to 300 Mbps" mode, use "up to 144 Mbps" instead if you want to choose a channel manually.
See https://community.plus.net/forum/index.php/topic,141577.0.html
Due to how the channels are spaced out, there are generally only 3 channels used that don't overlap each other: 1, 6, and 11. That's using the standard 20 MHz wide channels. Using the up to 300 Mbps mode tries to use 40 MHz of bandwidth, so it takes up twice as many of the available channels. So trying to use channels 4 or 9 would be about the worst possible places, since you take up almost all the available channels, without leaving enough room at either end, whereas setting 1 or 11 would leave more room at the other end for another network.

[#3]

Thanks very much for your reply. That's seems logical and reasonable now you've explained the physics.
Incredible therefore, that the developers didn't see fit to simply put a note next to the WiFi mode combo box informing of such limtiations and, even better, change the channel combo box so that only possible channels are selectable.
Never mind, nobody got hurt!