In supporting Wi-Fi networking adapters, I see it all the time - a user will say they can't connect to their access point, or they experience lots of disconnects. I get them to send me a diagnostic using our tool, which shows the wireless networks in range, and they are sharing a Wi-Fi channel with 5 other nearby radios, some of them showing up to 100% signal strength, according to Windows (which I know is all kinds of flawed, but still indicates a strong signal).
From my point of view, this is clearly the problem. Change your wireless channel.
From their point of view, this can't be the problem, as all of their other devices work just fine. It's just the one problematic device that has the issue connecting.
If I can get them to change their wireless channel to one that isn't used by other radios, then everything is usually fixed. Even then, though, the only explanation I can provide is that wireless interference affects different devices in different ways, which doesn't really cut it with some people.
I need a better explanation.