The signal largely comes down to antenna type, the piddly little ones that come standard with most ap's wont make it through less wifi friendly environments. Also you must remember that the type of antenna on these ap's is of the omni variety, and they generally have a poor radiation pattern both directly above and below the antenna, so signals going below and above the ap will always be poor compared to those located at the same height as the ap. I think this is probably where people go astray, and why I mention physics, because the waves do not travel in every direction at once and all of the same intensity.
I've done up some very basic diagrams of antenna radiation patterns, the radiation is indicated in pink.
A typical omni style antenna, as is found on most AP's and network cards, 10dbi
A typical omni style antenna, as is found on most AP's and network cards, 20dbi
A typical directional (eg: yagi, dish, panel) style antenna 10dbi
A typical directional (eg: yagi, dish, panel) style antenna 20dbi
The actual radiation patterns are more complex than this, but it's only meant to be a rough idea. You'll notice that at stronger amplifications, the signal spreads further allowing greater distance, but you don't get as much general coverage, anything outside that area will be purely hit & miss as far as signal quality goes.