@janeks 

If you notice the MAX DATA RATE on the dual channel ones is 360 vs 156, its down to channel widths.

The channel width will switch from 20mhz to 40mhz depending on what is connected too it at the time, when in 40mhz it will use 2 channels (7 and 9 in your case).   This is generally dependant on the device connected to the AP.

I notice your SSID is IOT so im guessing these are IOT devices?   From experience, 90% of IOT stuff has cheap and basic WiFi Cards are in them which are primarily 20mhz widths, the odd one supports 40mhz but its in the minority. 

You will also notice as widths get wider, the signal decreases due to interference.   Generally in 2.4ghz its recommended to use just 20mhz widths as its a very congested spectrum and when in 40mhz mode it floods 50% of it.    

I wouldnt worry about this, seems to be working as expected

  @Philbert Thx makes sense although it seems there is no setting to avoid 40 mhz

  @janeks 

Sadly not, it is on AUTO all the time so will decide itself when to switch between 20 / 40mhz

To be perfectly honest 2.4ghz is so flooded in general that you may a well use 40mhz to grab as much airspace as possible and hope for the best :)    5ghz pretty much solved that issue