[vdb86]

How are you determining it is not an RF issue?
I'd say it's either RF interference (being further from processor reduces noise)
Or it's RF attenuation (if wifi has to pass through other metal sockets then it will be attenuated, so "middle" sockets are worse than "edge" sockets.

Because I use usb extension cable (15cm) to connect the wireless adapter which is always at the same position with a clear line of sight in regards to router.
Physically how ever I plug-in that usb cable should not create any interference.