Signal variance is nothing new. I remember personally noticing it in late 90's that with my GSM (900MHz) phone, moving it on table just for about 5 centimeters could bring it down from full signal to no reception at all. Of course with analog radios you also notice how easily signal changes with location. If you've been ever playing with TV antenna in in bad reception and so on. Moving your hand in other room might block TV signal or make it crystal clear even if you would make there's no connection what so ever. With 2.4GHz people often forget that interference from other sources can significantly contribute. So signal quality and signal strength aren't same thing at all. Getting to the root all these things require professional, which I'm not. So one type of measurement defined as "signal strength" probably misleads you badly. Is it a good idea to select a wifi channel that doesn't have any other wifi boxes? Well, the reason might be that the channel is totally overpowered by local wireless CCT or phones. That's the reason why nobody's using it for WiFi and then you think it's a great idea to select a free Wifi channel? Radio stuff is (truly) really tricky. With higher frequencies it's just like light. Why some things are in shadows and some things are well lit?

I'd guess that it's due to signal reflections causing points of constructive and destructive interference.[0]

The shapes aren't very regular, but that could easily be explained by the shape of the room and its contents.

[0] http://en.wikipedia.org/wiki/Interference_%28wave_propagatio...

I found a great explanation from the reddit link that rawnlq posted.

http://www.reddit.com/user/Seventytvvo