In a nutshell, radiation strength is like wave theory. Drop two pebbles in a quiet pond and watch as the waves collide. Some will cancel out.
While you may think you only have one transmitter, the signal can travel multiple pathways and one spot/location can be a dead space.
As to performance differences in hardware, that's easier. Some parts work better than others.
I have two laptops with Linksys wireless adaptors, and a Westell Versalink 327 modem/router. We are in an old house with metal lath supporting plaster wals. One of the laptops is only 25 feet from the router, but sometimes the signal strength drops to 10 per cent and I cant get on the Internet. A few seconds later the strength is in the 20 per cent range, and in a few seconds more may either go up or down. I cant think of anything in the area that could be generating radio waves that would be intermittent. I have turned off the power to all appliances with no effect. Can anyone explain the constant variations in signal strength?
I am running XP Home on the computer showing these variations.